Since you have Vandersteen 5's already, I'd look at one place to start and that's Richard's M5-HPA's. I just got a pair and am floored by how great they sound. As others have said, you need to match the speakers and the amps to get the best performance.
I own Quatro's and will now sell my Ayre AX5/20 with built in crossover, because these mono blocks are so good. I've heard my Quatro's with the Ayre MXR's, Aesthetix Atlas mono blocks, Audio Research's 30k mono's, VAC's larger mono's as well as a few others and the Vandersteen amps are so far ahead of the others, that I'm floored.
They are also a great size and don't throw heat like my tube mono's always have. These are a very special amp as they have a ton of power for any room you will have your 5's in as they are a perfect match.