I have B&W Diamond Gen 2 for left/center/right and I'm driving them with Emotiva's. The Emotiva XPA amps are very good for the money, but they are kind of bright/harsh stock. If you want to improve things without a lot of money, I'd say put some upgraded fuses in your Emotiva monoblocks. Since you have a warm signature from the Marantz processor, I'd go with Furutech fuses to increase the bass punch and overall resolution. The XPA-1 monoblocks have two 10amp 5x20 fuses (on bottom plate) and two 1amp 5x20 fuses inside under the top cover. The Gen 3 amps uses circuit breakers, so no fuse upgrades for that (and I cannot comment on how it compares to old gen XPA-1 amps).
As far as amps go, I'm not sure I would go with McIntosh unless you want that really laid back sound (you already have a warm signature from the Marantz).
Classe is very good. Krell can be good, but it depends. You're looking in the right area. Need a really strong high current amp with big power supply for the B&W speakers. Maybe get another XPA-1 for center channel (if you think the XPA-1 are better than XPA Gen 3).