Preserving high-end L and R while going surround

If you are going to use minimal equipment for the center and rear channels, don't bother. Much of the adverse comment in audiophile circles about multichannel sound results from this practice. The notion that the rear channels are ambience only derives from early matrix multichannel experiments, where it was discovered that ambience in stereo recordings was often out-of-phase, and would isolate nicely to rear speakers.

Furthermore, long before there was discrete multichannel, I came to realize that the Center front channel, which in a stereo recording is signal that is in phase for the two channels, constitutes most of the signal. In particular, soloists are almost always recorded equally in the two stereo channels, so that in a stereo system they appear as a phantom source in between the two speakers. In a system that uses a center channel speaker, the soloist will be reproduced by that center channel speaker. The soloist is certainly important, and center speaker quality is crucial.

I agree in part and dissagree in part to the previous post. To achieve what you're looking for would require a 2ch. pre, with 2 inputs. One input for 2ch, then one as a piggy back going into the processor for Ht SS. When you switch inputs;you have either good 2ch--or HT. I do this,this way. (No ics or speaker wires to change--just the input selector) The quality of what you power the center and rears with/from?---Lots of good used amps;here. I use half of a decent 2ch amp for my center. Nothing matches in speakers or amps; but all sounds good enough for me in ss.

I agree that center speaker quality is of utmost importance. Ideally you want to have a center channel that is identical to your L and R speakers for a seamless sound across the front. If your speaker manufacturer doesn't make such a beast, I would inquire as to which of their centers would best match up to your current L and R.
If I were trying to cut the budget somewhere at this stage, I would buy a cheaper three channel amp with the same power rating as your 2ch.
Eldartford is correct, the phantom center of a 2 ch stereo will be reproduced through your center channel, therefore if your cc speaker doesn't match up to your L and R (at least in mid-range, treble, and sonic signature), then your entire front sound stage will be out of balance.
You will never know if there is a difference in amp quality if your center channel isn't good enough to convey it.