Hey, welcome to the Circles Freeturkeys!
A couple of comments for you, about your questions, and config.
1. I honestly have no idea what your gear is worth, so I cannot help there.
2. Why are you thinking of adding a 5.1 pre? Aren't you thinking of using the PC as the source, and pre and processor? Isn't that the point of HTPC, to use the soundcard as the pre and pro? IF that is the case, and I may have misunderstood you, then I would buy the Teac, and flesh out your surround sound, as you suggest. Heck, I would sell the Marantz gear and buy 2 Teacs - oh wait, I DID that...
3. Right, since you use the PC as music source, and I assume it has a DVD drive, it can be your pre/pro. The 5.1 card has outputs for 6 channels right? Tie those to the Teac and Marantz peice, and set the volume on those once, using the volume on your PC thereafter.
Make sense?
Mark in Canada.
BTW, there is a PC/HTPC Circle around here, where others, likely more educated than I, might be able to help you also...