Uhmm no idea. I would suspect the voltages because it would affect both cards, but you checked those already. The chance of having two cards with two problematic wavetable headers is rather low...? Can a grounding issue cause this?

I finally gave up after trying some other things like changing speakers etc without avail and thus replaced the X2GS with a MP3L (an external MT32 / Soundfont compatible synthesizer module). This works perfectly.

Then I put the X2GS in another PC which has an Orpheus I card. And big surprise: the exact same problem (so also very low volume). I now used 2 very different PC's and 3 ISA sound cards all with the same problem.

I really don't understand what the problem could be. If anybody else had some ideas left pleas let me know.

This is really strange. The returned X2GS was tested and had normal volume when testing here. Are you sure they didnt get mixed up ?
I can send you yet another one, i will pm you.