jtwrace,
The new Purity preamp that Bill (Bella Extreme/Response Audio) and I are coming out with shortly is excellent. I've been using the M300's for much of the voice testing. It's a perfect match but then it should be considering.
From a commercial standpoint, the Modwright 36.5 and 9.0 are very good choices. The H-Cat solid state preamp has worked well. SimAudio is another great combination. Tron is another one that's mated up well as well as Art Audio's and Gill Audio's preamps. Oh yeah, the VAC Renissance Signatuer preamp is also a wonderful combination and about the same price as the M300's. Actually a little more. We got very good reviews at RMAF the year before last with that combination and the Gershman Acoustics Black Swans.
The Clayton's are extremely versatile, your flavor of sound pretty much determines the front end but whatever you put in front of it, the Clayton's will bring out the best your front end has to offer.