What is the sensitivity of your B&W inwalls you have(or are considering)? Even if they're 90db sensitive, you will still do better dynamically with more guts under the hood(amp)!
Also, within reason, the kind of amp you use with an inwall system really isn't going to be the biggest factor! The biggest factor/challenge by far is going to be the "sonic limitations" that inwall speakers inherently offer.
Yes, for the most part,the inwall approach will keep your system sonic potential at a minimum, as those speakers have their challenges for sure! Usually, considering most aspects of the sound, you will not get the same sonic potential from $1500/pair inwalls that you will from $400/pr bookshelf monitors! Unless you build a "well engineered" enclosure for your inwalls, and EQ the heck out of em(plus other considerations), you won't get so fantastic of a sound in my experiences(and that's a lot). It would be ok, but not any better than even a lot of HT in a box systems for the most part.
The speakers are going to be the majority of the sound from your gear, and inwalls are your weakest link.
Might I strongly suggest, if you just do more of a surface mount for cosmetic reasons, that you go with the sonically supperior Thiel powerpoints!?! They're sonically superior flush mounts, which were judged as "best sound" at the 2001 CES show by the public, and are designed to go against the wall in a low profile!..it's a thought.
At least getting 3 of these upfront($2k used), and maybe downgrading the rears to inwalls will keep the overall sonic integrity of your system VERY HIGH! You will likely be spending more bucks than you originally thought, but not that much when you consider the 200% better sound you'll be getting...regardless of amp you'd chose for those inwalls you're considering(and the B&W inwalls are descent compared to others!)!
So, my vote is at least 3-5 Thiel powerpoints, and something like a used Parasound HCA1205a amp (used $600-700), Acurus 125x5 ($600 used), or similar(the Rotel $1k 5 channels amps are strong but descent sounding) and you'll potentially have more than fantastic sound for not too much!
Anyway, I'd recommend getting at least a better selection of fronts(again, powerpoints will amaze you compared to inwalls, or most other monitors for that matter), and you'll be WAY AHEAD OF THE GAME from the get go. good luck
Hey another choice is go to CES show in Vegas next month and check out the new stuff!