@mikeg Mike, my memory might be failing on this but I don't remember Cube "not liking V1s paired with 300B amps." What I do remember is a preference by Cube Audio for 45 and 2A3 amps. If you have more information on this from Cube, please share.
Based on my experience with the V1, the ~8 watts of output, typical of 300Bs, is (per my estimation) the bullseye for 'ideal' SET power to drive the V1s.
I'd expect a high quality design / execution of a 300B amp to be linear and neutral and be an effective pairing for the V1s. Chosen 'voicing' of the specific design, per the owner's preference, is an obvious factor.
If you remember, Cube Audio didn't like the pairing of the V1s with 300B based amps