Preserving high-end L and R while going surround

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.

Thank you for your sobering responses. I guess it would be best to just put together a separate surround sound from scratch. I'm thinking of the Definitive Technology Mythos series for speakers, with my Vandersteen sub. The Gramophone, a classical music magazine considers them "musical" as far as home theater goes; and then a solid 5.1 amp for around $1500 to $2000. Off to the net!