I figured that I was happier watching movies with my 32" Wega and 2-channels of acceptable audio than on friends' setups with bigger screens and lower-grade multi-channel, was used to a certain level of performance for audio, and would be disappointed by less in a multi-channel rig.
Along those lines, I haven't heard a receiver (whether 2 channels or more) that impressed me) so when I expanded to 8 channels (with the subwoofer) I decided to stick with separates.
My (now ex) favorite "high end" shop was unable ("we're not setup for that" Compare and contrast with the treatment I got on my stereo purchases, where they left me alone to A/B my top choices and even offered to send the top contender or two home with me if I couldn't make up my mind) or unwilling to let me A/B processors with the same speakers, shop #2 was worse (they actually didn't have anything beyond a used EAD in stock, and could at best order me something with no audition period), so I figured that buying locally wasn't a viable option and decided to get something used of the internet where I could resell it without taking a loss if I wasn't happy with the performance.
Lexicon processors seem to get universally good reviews that read along the lines of "completely neutral" or "perhaps with a hint of tube smoothness", have the proprietary Logic 7 modes for 7.1 speaker setups that are at the top or perhaps tied by Meridian (more on this later), and are very affordable on the used market so I figured I'd try a DC-1. To put it bluntly, Logic-7 in a 7 speaker setup is a quantum improvement which is almost to standard THX 5.1 decoding as 5.1 is to 2-channel. It provides a surprising you-are-there impression, much better envelopment, seamless pans to the back surrounds, and pretty much lives up to all the positive hype that's been said about it. If you're going to upgrade, go the distance with Logic-7 or perhaps the Meridian equivalent.
In the amplification department, I picked up some inexpensive used stereo power amplifiers (Adcom, although what you like, what you have/can audition, and your budget will make your situation different) that I was famliar with because
1) I knew what they sounded like
2) On paper, stereo amps often look better than their multi-channel counterparts because their higher idle current suggests a lower chance of encountering cross-over distortion and the bigger power supplies + heat sinks can't be bad (I'm a professional geek, and don't stop being analytical when I leave my office)
3) Such amplifiers seem to depreciate much more than their multi-channel brethern.
For speakers, I decided that I loved my main speakers and didn't want to replace them. So, I tracked down a second pair of identical speakers to use as side surrounds, a center with the same drivers, and pair of bookshelf sized rear surrounds from the same line. I think the full-range fronts+sides are a good idea - with flat response down an octave below the cross-over point, integration is a lot cleaner. I also believe the matching center is essential. Before I got the matching center, I tried a newer model with a different tweeter, found that it didn't integrate well especially on modes with soft or no steering on the front 3 channels. Provided that I was most interested arround the sweet spot, I'd have skipped the center if I couldn't find a direct match.
Finally, I added a Citation 7.4 passive sub-woofer. It plays loud low (-3dB @ 21Hz). The price was very right (they're being closed out. Over 50% off for new-in-box). It isn't boomy, although it could be a bit more neutral (I have it corner loaded which excites all room modes, so I'm going to apply parametric equalization before I blame it on the sub and not the room). Fairly tight too. In room response is arround 110dB at 22Hz, which is sufficient for my listening levels and a lot better than many powered subs at twice the price.
If I did it over again, I'd do the same thing with the processor (but might consider $2000 for a DC-2 rather than the $1500 I ended up paying for my DC-1), speakers apart from the subwoofer, and amplification. I'd consider a passive radiator sub to get rid of the port noise (I don't know that this is audible with real material and not test tones) or perhaps a sealed enclosure for that and a bit more tightness.
HTH. As a tangent, I think that the requirements for home theater and stereo are the same: accuracy. That said, good HT starts with good 2-channel.
While you could separate them, why duplicate the thousands of dollars you've spent on speakers and amplification?