I use my Moth 300B Paradigm, which cost me $1500 plus $500 for Western Electric tubes, and the only one I have heard and liked better is Min's Zanden 300B amp. I was looking forward to hearing the SDS-XLR with the K1Ks when my amp died, and I intend to try it out soon now that ironbut resurrected it.
The TTVJ/Millett 307A sounds great with K1Ks.
I also agree with Min on the WA5 being a good option and the Extreme not being a good option, but I have yet to hear the AirTight. Dussun integrated amps drive them pretty well, but not at all like the 300Bs I have heard.