Hi Ch1s,
after reading the above threads, I'd like to chime in a bit, regarding to your idea:
it has been a while since I'm doing basically what you want, without any issues.
As per attached shot, you'll find a mixer (Behringer Euro Rack 1832) plugged into a Denon 3806 AVR.
Care must be taken, however, if you want to play guitar in a loudy way; I mean, you must have PA speakers to be in the safe side.
My system is basically a full integrated one ( TV, AVR, DVD / DVR / VCR / CASSETE / TURNTABLE / MIXER ), so I can record/playback whatever I want thru the MIXER/AVR option. It's a no-brainer operation,though.
My speakers are all JBL Northridge E Series (not PA), but if you don't play the guitar loudly, you won't have any issues (I do have a separate guitar cube box anyway, which is not connected to the system).
Another point to mention is that it's not recommendable to have 2 equilizers ON at same time; I mean, the mixer has one, and the AVR has one too, so what I normally do whenever playing guitar is to turn off the AVR's EQ, because the Behringer's built-in EQ is far better for that purpose.
Furthermore, you can plug in some mics, pedals, etc., into the mixer, in order to have the desired sound effect.
I do set the AVR on STEREO mode and I'm done.
I've found this to be the best solution to suit my needs.
Hope this helps. If you need something else, just let me know, ok?
Cheers / Avliner.