As always, valves have a 'signature', but it's the circuit (particularly driver stages) and output transformers (or lack thereof) that make the sound. I've heard some 300B amps sound sublime and others a bit soft and ill defined. I have sparse experience of the 45, but I've listened to a number of 2a3 amps and again they have varied massively. The 45 push pull is an interesting idea, but PP designs inherently involve some cancellation effects, so I'd wonder how much of the magic dust will get blown away. My personal preference is to use one more powerful valve/circuit rather than two less powerful ones in PP or PSE.
As for the OP, it depends on how large a space you have and, more importantly, how loud you like to listen. The Avant Gardes have active bass, so are well suited to single-ended triode amplifiers in that damping factor will be less important. However, the quoted efficiency may still be marginal depending on your listening. I've heard AGs many times and come close to buying once or twice. The best combinations I've heard so far have all been with somewhat more power than would seem OK on paper. Rick has a pair and runs them with a 211, which would seem like overkill but actually works superbly. Others use 300B or even quite meaty solid state.
The Meyer amps fall into a rather special category. The build quality appears absolutely peerless, but you sure as hell pay for it. Buying new, you'd have to accept that you're either going to live with them for life or accept an absolute battering if you ever come to sell on. They would doubtless be relatively straightforward to repair if the worst ever happened, though if TM stops running his business at some point, you'd need to find someone capable of the same standards.
