I don't know much about the CAT amp, (other than Ken Stevens is a great designer, and his equipment sounds incredible!), but I have auditioned the VAC Phi 300, (configured as a stereo amp), at a friends house. It was a great amp, with incredible treble response, (the best I've ever heard actually), and a lovely mid-range, especially for vocals. (The bass was very good, especially so for a tube amp, although we both felt that the Lamm M2.2 (220 wpc hybrid) monoblocks were better.) I can highly recommend it. And running them as monoblocks will help the bass response, from what I have read.
One thing I will bring up, is that the VAC amp does run fairly hot. And running two of them will certainly raise the temperature in the room several degrees unless you have air conditioning. (Other than this slight annoyance, these are great amps, and certainly are reference quality.)
Good Luck in making a difficult choice!
