The Wyetech appears to be a very interesting model, although they will not have too many takers at $67k. I am not that up on technical aspects, but, the description which talks about an upper and lower leg sounds more like pushpull to me, with each leg in opposite phase. I have no idea how this topology compares with what is conventionally viewed as single-ended. As with any amp, regardless of tube type and topology, the proof is in the listening experience. Based on what I've heard of Wyetech gear, this should be a quite nice sounding amp; a friend had their 211 single-ended amp and I thought it sounded very good.
While I believe this thread is more of a theoretical discussion of why a particular tube can or cannot be used in higher power applications, I hope no one gets too caught up in any one particular topology or tube type as the be-all and end-all of tube amps. Even if one generally likes SET amps and generally likes the sound of 300B tubes, it is not necessarily the case that the best amp will tick both boxes. In the end, it really does come down to auditioning the particular amp.
I personally like SET amps myself and own a parallel SET amp (Audionote Kageki). But, I also own a pushpull amp that uses 349 pentode tubes and it is the amp I currently have in my system. There are tradeoffs both ways in terms of the sound of the two amps. For example the pushpull amp has a tighter and punchier bottom end, but the SET amps bass sounds more subtly variable (i.e., less mechanical sounding). But, the best sounding amp I've heard is a custom built 30 watt OTL amp; I did not consider purchasing it because it is a bit frightening (massive, complex startup procedure, no protection circuitry so there is a risk of blowing up one's speakers). Hence, I don't consider any one topology to be the absolute best--it really depends on the particular implementation.
As for triode tube type, the 300B is probably the most popular. I suspect that a lot of that popularity comes from it being the most practical tube in terms of output. The other small triode tubes that don't require extremely high B+ rail voltages just don't put out enough watts compared to the 300B. Yes, there are a lot of people that flat out prefer the sound of the 300B to other triodes, such as the 45 and 2a3, because of its rich upperbass/lower midrange, but, most of my friends with high efficiency speakers like the 45 tube more (tighter bass and more extended top end). I like the 45 too, as well as the 2a3.
To add further to the complications, different tubes under the same type can sound quite different. A Kron 300B will sound quite different from a Western or a TJ 300B.
If higher power is needed, it makes sense to consider all options: higher powered SET amps, such as the 833 mentioned above, or 845, 211, GM70, or 1610 tube; pushpull amps and OTL amps (I particularly like OTLs in the power range we are discussing) and even solid state amps. In short, avoid the trap of only looking at certain tube types and topologies.
