Network captures from Denon X1500H 25 Aug 2019
Commands sent with Denon 2016 AVR Remote Version 3.2.0
on an iPhone 6

The endpoint is /goform/AppCommand.xml unless the entry is 
tagged with 0300, then it is /goform/AppCommand0300.xml

The 0300 endpoint seems to correspond to the markup <cmd id="3">
whereas the regular endpoint seems to correspond to <cmd id="1">

My Wireshark filter (set destination to your AVR's IP address)
http and xml and ip.dst==192.168.0.119 and !(xml.cdata contains "GetAllZoneSource"  
or xml.cdata contains "GetAllZone" or xml.cdata contains "GetActive" 
or xml.cdata contains "GetInput" or xml.cdata contains "GetVideo")
Eventually I got annoyed and simply set:
!(xml.cdata contains "Get")
though this will need to be revisited for updating state.

A Linux VM was sufficient to handle MITM:
sudo sysctl -w net.ipv4.ip_forward=1
sudo arpspoof -i eth0 -t 192.168.0.101 -r 192.168.0.119
    where the first IP is your phone and second IP is your AVR

Command structure:
<tx>  <-------------------------- begin command
  <cmd id=%d>  <----------------- id="1" for /AppCommand.xml endpoint and    
                                  id="3" for /AppCommand0300.xml endpoint
    <name>Set%s</name>  <-------- %s CamelCase string to name target ex
                                  SetSurroundParameter
    <list or value>  <----------- For simple binary commands or setting
                                  levels value is used.

Example list:

<list>  <------------------------ set the parameter "dynamicvol" to 0
  <param name="dynamicvol">0</param> 
</list>

Example value:

    <value>24</value>
  
  </cmd>
</tx>

Upon success the AVR replies with: (needs more testing)
<rx>OK</rx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetAllZonePowerStatus</cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetSourceRename</name>
    <list/>
  </cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetZoneName</cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetSoundMode</name>
    <list>
      <param name="movie"></param>
      <param name="music"></param>
      <param name="game"></param>
      <param name="pure"></param>
    </list>
  </cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetAllZoneSource</cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetAudioInfo</name>
    <list>
      <param name="inputmode"></param>
      <param name="output"></param>
      <param name="signal"></param>
      <param name="sound"></param>
      <param name="fs"></param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetInputSignal</name>
    <list>
      <param name="inputsigall"></param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetActiveSpeaker</name>
    <list>
      <param name="activespall"></param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetVideoInfo</name>
    <list>
      <param name="videooutput"></param>
      <param name="hdmisigin"></param>
      <param name="hdmisigout"></param>
    </list>
  </cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetAllZoneVolume</cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetAllZoneStereo</cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetAllZoneMuteStatus</cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetECO</cmd>
</tx>

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">GetQuickSelectName</cmd>
  <zone>Main</zone>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetAudysseyInfo</name>
    <list>
      <param name="eqname"></param>
      <param name="eqvalue"></param>
      <param name="dynamiceq"></param>
      <param name="dynamicvol"></param>
    </list>
  </cmd>
</tx>

The following command raised the Center channel "C"
by 2dB.  Lowering back to 0 was value 24.  
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">SetChLevel</cmd>
  <name>C</name>
  <value>28</value>
</tx>

Confirmed that -12dB = 0, 0dB = 24, +12dB = 48
Confirmed uses conventional names:
FL, FR, SL, SR, SW, C (probably RL and RR for 7.1)
Someone would need to check Atmos naming conventions
but manually setting Atmos channels isn't really desirable.
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">SetChLevel</cmd>
  <name>SR</name>
  <value>24</value>
</tx>

Enable Loudness Management for Dolby TrueHD
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSurroundParameter</name>
    <list>
      <param name="loudness">1</param>
    </list>
  </cmd>
</tx>

Disable Loudness Management
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSurroundParameter</name>
    <list>
      <param name="loudness">0</param>
    </list>
  </cmd>
</tx>

DCO setting can take 0: off, 1: low, 2:med, 3: high
This setting will only work for Dolby tracks that contain
the dynamic range metadata but the effect is much better 
than traditional dynamic range compression.
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSurroundParameter</name>
    <list>
      <param name="dyncomp">3</param>
    </list>
  </cmd>
</tx>

Low Frequency Effect takes settings from 0 (normal) down 
to -10 (-10dB).  This is a useful addition for night mode
macros.
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSurroundParameter</name>
    <list>
      <param name="lfe">-10</param>
    </list>
  </cmd>
</tx>

From here on we are assuming that the 1 (on) and 0 (off)
syntax will remain consistent.
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSurroundParameter</name>
    <list>
      <param name="cspread">1</param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetAudyssey</name>
    <list>
      <param name="dynamiceq">1</param>
    </list>
  </cmd>
</tx>

Ref Level Offset (3 = 15dB 0 = Off)
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetAudyssey</name>
    <list>
      <param name="reflevoffset">3</param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetAudyssey</name>
    <list>
      <param name="multeq">1</param>
    </list>
  </cmd>
</tx>

Dynamic Range Compression when Dolby DCO
is not supported
0=off, 1=light, 2=medium, 3=heavy
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetAudyssey</name>
    <list>
      <param name=dynamicvol>0</param>
    </list>
  </cmd>
</tx>

Dialog Level (center channel adjust) from
0 (-12dB) to 48 (+12dB)
Note that this was probably calibrated by Audyssey.
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">SetDialogLevel</cmd>
  <value>17</value>
</tx>

Would you like to rename a source with Python? ;>
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSourceRename</name>
    <list>
      <param name="DVD">Test</param>
    </list>
  </cmd>
</tx>

0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetFirmware</name>
    <list>
      <param name="timezone">38</param>
    </list>
  </cmd>
</tx>

Quick Select takes values 1,2,3,4
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">SetQuickSelect</cmd>
  <zone>Main</zone>
  <value>1</value>
</tx>

Change input 
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="1">SetInputFunction</cmd>
  <zone>Main</zone>
  <value>TV</value>
</tx>

Enable Pure Direct
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundMode</name>
    <list>
      <param name="genre">4</param>
    </list>
  </cmd>
</tx>

Pure Direct "Direct"
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">1</param>
    </list>
  </cmd>
</tx>

Pure Direct "Pure Direct"
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">2</param>
    </list>
  </cmd>
</tx>

Pure Direct "Auto"
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">3</param>
    </list>
  </cmd>
</tx>

Set to Movie Mode:
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundMode</name>
    <list>
      <param name="genre">1</param>
    </list>
  </cmd>
</tx>

Set to Stereo Mode:
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">1</param>
    </list>
  </cmd>
</tx>

Dolby Digital
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">2</param>
    </list>
  </cmd>
</tx>

DTS 
0300
<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>SetSoundModeList</name>
    <list>
      <param name="listno">3</param>
    </list>
  </cmd>
</tx>

Fetching available surround modes
This command will give you the surround mode list available for current selected genre
(music, movie, game, pure)

0300

<?xml version="1.0" encoding="utf-8"?>
<tx>
  <cmd id="3">
    <name>GetSoundModeList</name>
    <list>
      <param name="genrelist"></param>
    </list>
  </cmd>
</tx>

and you get a response like this

<?xml version="1.0" encoding="UTF-8"?>
<rx>
   <cmd>
      <name>GetSoundModeList</name>
      <list>
         <param name="genrelist" control="2">2</param>
         <list>
            <value>
               <listno>1</listno>
               <dispname>Stereo</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>2</listno>
               <dispname>Dolby Audio - Dolby Surround</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>3</listno>
               <dispname>DTS Neural:X</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>4</listno>
               <dispname>DTS Virtual:X</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>5</listno>
               <dispname>Multi Ch Stereo</dispname>
               <selected>1</selected>
            </value>
            <value>
               <listno>6</listno>
               <dispname>Rock Arena</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>7</listno>
               <dispname>Jazz Club</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>8</listno>
               <dispname>Matrix</dispname>
               <selected>0</selected>
            </value>
            <value>
               <listno>9</listno>
               <dispname>Virtual</dispname>
               <selected>0</selected>
            </value>
         </list>
      </list>
   </cmd>
</rx>