They're being driven with a Pass Labs INT-60 integrated, which is supposed to be 120 watts into 4 ohms. My experience is that this works well, I can get near 100 db in my room, which is more than enough for me, but that would be pretty much all cranked up. Were I to do this again I might go with the INT-250 but I'd have to say I'm happy where I am. The sensitivity is fine for most applications but you're correct about the impedance. Most of my speakers in the past have been dipoles or open baffle and I wouldn't mind doing that again (with way fewer watts and tubes, likely), but I'd have to say what I have now is probably the best overall the cleanest sound I''ve had so far.
KEF R7 META
Those of you with KEF R7 Metas: how happy are you with them and how are you driving them?