Wes,
The components listed are all pretty good pieces of gear for their time and class. However, that SP-91D processor is way behind the times in terms of surround decoding capabilities. By that I mean it will only decode the analog Dolby Pro Logic format. It will not decode any of the newer digital formats such as Dolby Digital(5.1)/Dolby EX(6.1)/DTS(5.1)/DTS ES (6.1), or Dolby ProLogic IIx(7.1). These newer formats, even while most having been around for years, are light years ahead of the old Pro Logic system in terms of sound quality.
If you are really wanting to get into a home theater setup you would be doing yourself a great injustice to stick with that processor.
My .02 would be to ditch the C-91 preamp and the SP-91D processor and move up to a newer preamp/processor that has the newer surround decoding features. Now, that C-91 preamp is a pretty decent item so if you wanted to set up a second stereo-only system, you could hang on to it and make some use of it. The processor though, sorry to say, is probably a disposable item - not much value unless it's sentimental. You could try to sell it, but I doubt it'd pull much more than $50, if that, could be wrong.
The power amp is a 2ch/4ch design so it could be used in conjunction with the C-91 for that 2nd system 2-ch only, or could be used for the surround channels in a newer setup. If you opt to use it for a 2nd 2-ch only setup, then you would need a new/used amp with 5, 6, or 7 channels to use with the new/used processor. If you keep the power amp to use with the home theater, then you'd need a 3-ch amp for the 2 front speakers and the center channel.
If all that sounds overwhelming, you may just want to consider selling (or setting up that 2nd 2-ch system) most of the Pio stuff and getting a newer surround sound receiver. Based on your description of intended useage, that may be the best way to go anyhow. There are some very capable surround receivers these days that don't cost a fortune unless you want one with the audiophile name and performance.
Regards