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.