Best used five-channel amp?

I'm looking to upgrade my trusty Primare A30.5 amp to something a little more muscular a higher end.  I have a Classe Sigma-SSP processor and B&W 804s units for the front and an HTM3s for the center.  Does anyone have recommendations for used units?  I've been thinking about the Classe CAV 500 or the Theta Dreadnaught.  I'd like to keep it under $4,500 or so.  Thanks!
NAD M17, it's a beast.  I know it's a seven channel but you can bi-amp;). Anyway, I am using it w/the new 804D3 in a 7.1 setup and it makes those beauties sing!
I don't know about "best" but an Anthem P5 is a beast at 325/ch.  New around $8000.00 used in the $4000.00 vicinity.

If you can find one, CAL (California Audio Labs) MCA-2500.  It's a beast and can put out $500w/chan.  

In your 2 choices, I would lean towards the Classe CAV 500.  It definitely has a much beefier power supply.  Since you are looking at the "older" 5-channel amps, I would also suggest looking at the Krell TAS.  There's one up for sale for $3098.  The Krell has more power supply and brute force then the Theta.  I don't know how it compares to the Classe.

I have the B&W speakers too, and you really want a high current muscle amp to make these speakers sing.  Look for massive transformers and capacitor banks in your amp. 

The Theta Dreadnaught has big transformer, but smaller power supplies on each amp board.  The Anthem P5 had very small individual transformer/power supplies on each amp board.  In my experience, smaller power supplies mean less brute force in the bass/midbass area.  Please note that these are not bad amps, but the huge power supply amps have an edge in low end muscle/body.

I don't know your space requirements, but another idea would be to look for a really hefty 3-channel amp and then use your Primare for surrounds.

Please don't kick me here for this next suggestion, but I have found that the Emotiva monoblocks mate very well with the B&W speakers.  I'm using big XPR-1 monoblocks for each of my three front channels and smaller XPA-1L for surrounds. 

If you had space, you could buy five of mid-level XPA-1 monoblocks and do Isoclean fuse upgrades.  Alternatively, you could look for an XPR-2 and XPR-3 if you only had limited space (also with Isoclean fuse upgrades).  Either of these options fall within your $4500 budget. 

Thanks everybody!  I wound up snagging a CAV 500 for a very good price.  Question, though:  I was planning on placing it in my entertainment cabinet, which has held my Primare amp without a problem.  My concern, though, is that while the amp will have plenty of overhead clearance, there will only be roughly an inch of clearance on each side.  One side has holes drilled for ventilation, but the other is solid.  The back is open.  What's the risk here, and is there anything I can/should do to mitigate it?  It's probably impossible to place the amp anywhere else--challenging room.
I don’t see a problem there. Heat rises. An inch is fine on the side if you have good space over the top of the amp.  Let us know how it sounds!
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Bryston 9B-ST.
Krell theater amp standard AKA TAS used them on my 804n and now my 803D no problem
Lots of "beasts" in this thread.  Every amp is a 'beast"?  Okay... my "beast" suggestion - Balanced Audio Technology VK6200.   Each 200 watt channel is a completely separate amplifier with its own power supply.  In its six-channel configuration, the "beast" I have weighs 200 pounds.
Cal Audio Labs CL2500-MCA, Theta Dreanaught, Krell 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.
I went citadel 1.5 on my snell xa90ps, and a sigma amp5 for my center and surrounds here the combination is a match, and sound phenemonal with music and movies(and is fast), granted i have active subs, and a smaller room, but it works and works well, and is not overkill by any means..and dont knock the class-d.
My beast suggestion is the verastarr ssa644