Newbie question about amps and sound cards

Well, when I blamed the soundtrack at the beginning I was talking about the distortion that I had when I changed my PC, but as I said before, that happened to be due to an ungrounded outlet and is now solved.

On the other hand, I think the difference in dynamic range from the PC setup to the amp setup is probably due to limitations of the PC's speakers. I have always considered my speakers to be relatively "good" because they are much better than the crappy ones that typically ship with PC's, they can be quite loud and have a subwoofer that makes them hold up well through bass beats. But I guess that for making music, quality matters more and one sees the difference better.

Therefore I think I will still use the PC for recording as you say, since I don't have any real reasons to suspect the soundcard now. Something that would be interesting to do would be connecting the theremin to the PC, and then the PC's line-out to the amp's input, so that I can use the PC for recording and at the same time hear myself with good quality. But I'm not sure whether one can connect the PC's line-out, which is stereo, to the amp channels (or one of them) which are mono. I have googled this but some people say it's not problem and others say I'll short-circuit something... :S

Not a problem. I go from stereo to mono quite regularly and have not short circuited anything.

Google [i]stereo to mono adaptor[/i] and then narrow it down according to what sort of plug you want - 1/4 inch jack, 3.5mm jack, RCA plug.

I got a stereo minijack to 2x mono RCA adapter, and then a mono RCA to mono jack adapter, it was the simplest "path" that they had in my local shop. I connected the right RCA cable from the PC to the amp, ignoring the left one, and it works perfectly; although at some point I'll probably buy something simpler at another shop or online so I don't need to have a dangling unused RCA connector.

I found an effects processor program for the PC, Guitar Rig 4, that has a free version that can apply reverb and delay. I suppose it's not like having the real physical thing, but I like how the theremin sounds with some reverb anyway.