I'm using a Primare P30 pre/pro in my 2-channel system. It has a 100% analog by-pass mode which really helps sonically. I've compare it with an Ayre k3-x preamp in my home and the Primare was signifanctly better in most respects. It retails for $4k but you can probably do better price-wise.
There's probably a number of reasons that a good 2-channel is usually better than a multi-channel setup. I'll take a stab at a few reasons:
1. Perhaps because it has to do more, multi-channel equipment historically has usually been sonically inferior when compared to traditional 2-channel equipment. Though that is changing.
2. An excellent multi-channel system would cost significantly more than an excellent 2-channel system. Therefore, if one had a budget of say $10k, the sonics would hopefully be far superior in the 2-channel because one would be able to afford better quality 2-channel equipment than spreading that budget out for 5 pair of cables, speakers, amplifiers, etc..
3. I just don't know why. I've listened to some very pricy surround systems costing $50k or more and it just sounds, shall we say, different.
So I figure, why hassle with the added expense and headache of all those cables, speakers, etc. and just enjoy what one has.
I believe that if you purchase the right 2-channel equipment and setup your system properly, you'll be in want of nothing.
Besides, life's pretty complicated as it is.
-John