Skillzo1
Been watching this thread progress for some time and it has suddenly dawned on me. My volume level wasn't all that when I got my Dudu after tinkering with various settings (it was louder when I first turned it on, before tinkering).
So, I reset all the sound settings to factory defaults that I could find. The audio volume was restored and is now plenty loud enough. Even though the amp in Dudu is around 25-35W @4ohm it powers my 60W Focals (installed by myself, not factory fitted) just fine. Vol 15 is plenty loud enough for me, like others report.
As you have come this far and tried many other things, have you tried resetting all sound settings to default and starting from scratch? Could deffo be worth a go. Also, before you do this, I take it you're aware that you backup your sound (and launcher) settings under the 1st option of Dudu, Settings/Account Connedction/Cloud backup... Just in case you do find there's no difference and want to restore your original setup.
Another thing you can try that could well make a difference also is to open the Dudu app Sound Master (or grab it from the Dudu app store if it's not pre-installed, I can't remember) where people have uploaded various equaliser etc settings of their own configuration. I found quite a few of these added a fair amount of audio gain.
One more thing to note: I think you said you were running aftermarket speaker that your guy fitted, correct? If so, has you guy got an oscilloscope for measuring gain eg with a plain sine wave at 1000Hz playing to measure for clipping?
The reason I ask is because I too tried a similar amp to yours, plug and play with 16pin connectors. Trouble is, the 2nd amp isn't really getting any more power direct from the battery (that I know of, i'm no expert). It's just cranking up the gain of what's available on the existing power circuit. Now, you don't need to go as in depth perhaps, but I bought 2x dummy load resistors of 100W at 4ohm, disconnected the speakers and placed the resistors to represent the load of the speakers. I then hooked up an oscilloscope and played a 1000Hz sine wave from a good quality WAV file via the Dudu. What I found was with the 2nd amp, similar to yours, although there may have been more 'loudness' it came a t cost of severe clipping to the sine wave.
If your speaker setup is/was quite expensive, continuous clipping can eventually damage speakers (or so I have read, like I say, I'm no expert on this). Worth mentioning though as I wouldn't want you to damage your speaker setup because of a poorly implemented 2nd amp setup. If a 2nd amp is the only forward for, having also tried my initial suggestions of resetting etc, you might want to consider a 'proper' amp that draws direct from the battery and with it's own ground.
I sent my two 2nd amp options back to Aliexpress after seeing how bad the clipping was when gain was applied to either of the 2nd amps.
If you're not sure what I mean by clipping, see img attached.... Basically, you have a nice clean sine wave that's all well rounded that shows a clean audio frequency, but if too much gain is applied then clipping occurs where the wave form becomes squared off - clipping the peak and trough of the frequnecy curve. If that makes sense!!
