7.1 HT under about 3k

This shouldn't really be a big deal. Aragon STage One feeding some used 5-7channels of amplification, used, at under $1500 for amp, and $1500 for pre should not be a problem. You could also go Anthem AVM20 used at $1500 range maybe, and I'm sure others. $3k should be easy on the "used market" for separates

"... My comment on not going 6.1/not placing rear speaker(s) in the center of the back wall was to avoid possible front to back image reversal. There are a lot of postings on this at avsforum." (Bruceomega)

For the record, as someone who does acoustics, I think anyone reading the above might be getting some mystical acoustical pheonominon worries here, that simply aren't going to happen in any real world scenarios! Basically, distances and delays in the system, acoustics in the room, recording mixes/delays are going to make what Bruceomega's infering a near impossibility, as a problem!
I've done hundreds of systems/installs over the years in 6.1 or more, and this is not an issue, trust me!
From gathering what Bruceomega is talking about, again, all the right varriables, elements, adjustments, acoustics, and recorded material would have to be in precise allignent to make you think you were hearing the back info in front, and the front info in back! You'd also likely have to not only have lousy acoustics (in which case, you can forget about good sound anyway), but you'd probably need to have the processor in your system set to "Concert Hall", er "Stadium" or whatever!!! Last I checked, most people doing discrete dd/DTS use very little additional DSP modes with their sound! It's already mixed as well as it needs mostly, if you do things right...you needn't even go there!
Anyway, as long as you have either some diffusion or absorption either on your back wall, side walls, or front, and preferably a combination of those, you'll never encounter any mystical problems like what's being infered above, I assure you.

Joncourage, I presume you're refering to the part about "looping" your 7.1 processor into an auxilary input, or bypass input, or whatever, on your 2 channel preamp?
Basically, you do just that, to keep your separate 2 channel system, well, separate, for 2 channel dubties.
It's simple...when you use your 2 channel setup, you keep things all connected as normal. The CD player, tuner, record player, whatever, all get plugged into the back of your existing 2 channel preamp. The pre-out's of your 2 channel pre connect to your amp, like normal...nothing different. However, for your 7.1 preamp/processor, you would connect the main L/R out's from that, into the above mentioned "auxilary" or "bypass" inputs(you could probably use a "tape-in" as well, I suppose) on your 2 channel preamp, just like that input was from another source.
You'd then connect all outher sources, like digital connections, to your 7.1 pre/processor, and connect all the "pre-out's" for the other channels(center, sides, rears, subs), to that 7.1 pre also, out to the other amp channels. Then, when you want to watch movies, you simply switch to the "auxilary"/"bypass" or whatever input on your 2 channel preamp, which plays through that preamp for L/R channel, and of course plays the other channels as normal from your 7.1. It's just that the main L/R channels is playing out of the 7.1 pre, into the 2 channel pre, and out to it's amps. You'd then set the 2 channel pre at like 12oclock(somewhere in the middle likely), and let those 2 channels play through. Then, when you listen to music from your 2 channel sources, you can simply switch off the 7.1, and use the 2 channel set up as normal.
Basically, you're using the 7.1 system as another input going to the 2 channel system!...only you're having to connect all the other channels to the 7.1 pre, as if the 2 channel pre isn't there. Do you get that? If not, someone can send you a diagram, I'm sure. Hope this helps