I'm wondering if the DSP affects the sound quality (on either rear or front channels), or does it just copy the signal without quality loss to the rear?

It directly copies the front channels to the rear.

There are some cards, typically 2 channel only ones, that perform special post-processing on >2 channel audio streams when converting them to a stereo audio stream. Again, a driver issue, but it has adversely affected audio in my experience before.

So, the answer is that there's no audio quality difference in fb2k.

And how about using it with CMSS? Does it collide with it?

I don't know what CMSS is... All I know is any of these pseudo-3d post-processing effects added by soundcard drivers can kick in when down-sampling >2 channel material to 2 channels, and I also know that I don't like that.

I think CMSS is for expanding two-channel material to 5.1.

CMSS is 2->5.1
If you use foobar's 2->4 then it would either really screw with the cmss, or the cmss would not kick in depdnding on how your drivers do the cmss, ie if they are smart and don't cmss things with more then 2 chanels, no problem =)