dB Flexsys FM12 distorted sound

[HellsBells]

I own a couple of active stage monitors dB Audio Flexsys FM12 which are 5-6 years old.

One of them now shows the effect that any audio signal (except for super low volumes) is heavily distorted and sounds like crap. Everything else seems to work (I can increase the volume, switch the DSP settings,…) but the disortion is permanent.
I already swapped the speakers from one of the “good” devices into the faulty one but that did not make any difference, so I can rule out the speaker.
I opened the cabinet and removed the PCB but could not find any obvious damage (like a leaked capacitor). I measured the main accessible components with a multimeter and they seem to be OK and now I am stuck.
The PCB is labelled PST440A and I could not find the schematic online, only one for the PST427.

Any idea on where to search for the „error“ or if someone else had the same issue?

[Jazz P Bass]

You could try to narrow it down to an input preamp problem or a power amp problem.

[HellsBells]

Thanks Jazz P Bass, any suggestion on how to best do this? I do not have a schematic of that particular model so that seems to be a tough one

[x-pro]

Based on the PST427 circuit, find a power amplifier input, apply a signal there, and listen to the response. Better, of course, is to use a generator and oscilloscope with a resistive load.
There in the aluminum box is a circuit for the amplifier on the TDA8950TH or TDA8954T chips