Do I need to amplify all of my channels?

Gurus:

I have recently moved to a new house that has a media room, into which I am trying to harmonize my current equipment. My existing setup consists of a McIntosh MHT-100r, a pair of Bowers and Wilkins 803s, and a Bowers and Wilkins HTM2 center channel (wall mounted and left in the old house were the surrounds - decent klipsch).

My new media room includes high end front in-wall speakers (not audiophile but top of the consumer range that I may move to sides or rear), above average consumer in-wall sides and rear speakers, and a decent wall mount Sonance center channel that I will sell. There is also a solid Velodyne sub that I intend to use (I am writing from an American Airlines inflight wifi connection and cannot remember the model).

I have long believed that my MHT-100 (80w at 8ohms) is underpowered for my B&Ws. My desire is to add an amplifier to my system, but I don't know how many channels I really need. The MHT-100 has 5.1 incorporated amplified channels, but can pass 7.1 (for which my media room is wired) to an amplifier. The MHT does not have balanced outputs.

My primary goal for this room is media, though it is the best equipment in my house - so i will listen to Andreas Vollenweider Caverna Magica on this system.

So the questions are: do I just need 3 channels of amplification for my B&Ws or do I need 7; if I use an amplifier for just three channels, how does the system work (ie volume control) and how do I hook it up; if I use a 7 channel amplifier, will I overdrive my sides and rears (they are rated at 100w, I think); and any other questions I haven't considered.

I'd also appreciate any options that amyone would like to suggest, such as "get the Outlaw 7 channel 200w" or "McIntosh MC-x is the only way to go with the equipment you have." I am trying to do this on a smart budget, as media room furniture is also needed and money isn't endless. However, I want the right equipment so I don't have to buy it twice.

I appreciate any feedback/suggestions/advice/criticisms.

Thanks in advance,
Mark