I have to disagree with you on the surrounds. There just isn't enough information going through them for someone in the middle of movie to actually have the difference in tonality yank them from the immersion.
That money is better spent on being put into the subs. Same with the amp. If that A51 is going to end up running $2500+ I just see that is a poor return on investment. Those PSB's will sound great with any competent amp.
I agree with the receiver suggestion. You could simply run the surrounds off of the receiver amp section. Wouldn't break a sweat.
We will just have to agree to disagree on what best is.
There's a lot happening in rear surrounds at this point compared to when surround sound 1st became main stream. Add to that all of the different modes of listening that allow the rear channels to produce sound and I feel that to design the optimal system the rear speakers should be treated better than a footnote.
Add to that the desired volume that the OP is saying he wants to listen at the rear channels will be taxed. So, IMO he needs something that will handle the stress. $90 in-walls will not do so.
Also, looking at the pics he posted, I'm not too sure how in-walls will lay out in there.
If someone is going to buy $5,000 mains they care about the best sound they can get. The best would be to use the Synchrony S as the rear surrounds. To not do so and go to entry level rears would be like buying a Corvette and then going to Goodwill to scrounge around for used tires. You're not getting the performance that you obviously want.
We just did a system using Phase Technology rears that weren't all alike, but their driver materials (tweeter) were the same. Woofers were different and you can't tell at all. I would use these speakers with any of the Phase Technology PC product line as they were designed to be used that way.
At loud volumes, like whentrying to recreate a movie, I think that the most seemless transition from front to back is best. Keeping the speakers in the same family allows that.
Lots of different high quality amps will work. The PSBs will do great with the Parasound New Classic models as well. I had a pair of 1s for demo from our rep and used a Parasound 5125 and they were excellent. I would've kept them accept that the room I do stereo in has a lot of furniture and will not allow me to permanently place them where they would sound best. My room is narrow 12' across and if I were to get a pair I would probably want the Synchrony 2s. Where i would have to put them, the 1s would produce too much bass. Besides that, in my room I liked the Phase Technology PC 9.5 better:
http://www.phasetech.com/products.html?product_id=PC-9.5# and the Cherry finish on them goes better with my fireplace.
But that's why this hobby is fun! Different things are available to do different ways and suit everybody!