You could always attempt to disconnect what look like JP1 and JP2 and then reconnect Pin 1-2 (Instead of 2-3 as it appears now) to get Line Out, or wire Pin 1 on each directly to a cable and see if the problem goes away as that would eliminate some potential problem circuitry.

I just soldered audio jack to preamp pins, and, what's interesting, everything sounds fine when only one channel is connected. When I connect second channel, all the bass goes away...

Perhaps the ground is not connecting properly or the signal pins are connected together somewhere?

Can you confirm that the card has working stereo output? That MP3 makes it look as though this is not the case.

Yes, i tried panning audio channels in windows mixer and it works fine.