Best used five-channel amp?

Cal Audio Labs CL2500-MCA, Theta Dreanaught, Krell HTS...in order.

Got an Anthem P5 4000, 5 yrs old. Killer. Best I ever heard and had.

Don't discount the PVA series amps from Anthem. Super clean and it will drive the 804's without any issues. Only omission is lack of balanced inputs, but if that's not a must, you can pick one up really inexpensive and I bet you'll be pleasantly surprised. It's definitely a "hidden bargain" in the amplification world.

I purchased a pristine Classe CA 5300 Delta series amp to go with my Classe SSP 800 about 3 years ago for $4900 .  I would think 300 X 5 should be enough for nearly any home theater. It certainly is plenty in my case.  I love the sound, but truth be told it did have to go back to Lachine for repair once.  The good new is Classe paid for the repair even though I was not the original owner, but the amp was still in the warranty period.

If you want something that is going to sound nice with B&W Speakers take a look at the signature series ATI class ab amps or the Hypex Ncore  ATI  class d amps.  Before you scoff at the ATI class d amps read some reviews because I know a lot of people do not like class d but the Hypex ncore’s are nothing like the old Ice power class d amps.  The ATI class D are also getting very good reviews and are resonablily priced.