good 5 channel amp for rears only

You really should try to match the center to the right and left. Many Classe amps are bridgeable. Get a stereo amp for the center and either bridge it or biamp. Then, pick up whatever for the rears/surrounds. You will much happier this way.

Elevick has a good point. You want to match your front 3 speakers as best as possible or it will sound off.

YOu've a good synergy for the money with your classe/B&W combo. Your main concern with "timber matching" is already adressed with the speakers. Slightly differing amps won't matter too much in the long run, especially considering how movies are mixed. That said, a tonally similar yet strong match would be the dirt cheep(used) Parasound HCA1205a. Strong enough to driver your speakers, and a good sounding HT amp, good enough to do justice to your B&W's for your purposes I think. I owned the Classe CA100/150/300, and am familiar. I've sold the B&W's for years, as with the Parasoujnd stuff.
A slightly more "extended" choices is the ATI 1505 amp, but I like the Parasound in your case.
Anyway, I'd also like to see you in a sonically superior pre/pro than the b&K!..which is just OK. The Classe SSP's, Krell's, Anthem AVM20, Aragon STage One, and others stomp the sonics of the B&K from the digital in!
Anyway, good luck.
Also, may I suggest strongly running your B&W's as "small" on your pre/pro!? With a powered sub, you'll get much much better dynamic ability and control/efficiency with your sytesm, as THX predicates. Running as "full range" through your passive speakers, dynamic output and control will be limited, and bass modes hard to control from 7 speakers in all but the largest of rooms.

I agree with the previous post in that you have addressed your timber match in your speakers. I'm doing the same thing in my system only using 5 channels instead of 7 in HT mode. I think the Parasound is a great way to go as I am using the 2203 which is a three channel amp for my surrounds and center. Parasounds are very resonable, very reliable, great customer service and do not call attention to themselves. Yes, you can find a 2205 dirt cheap. You are on the right track in my opinion.