Well, IMHO the only thing 'wrong' with single ended amps is the power on offer. You have to have efficient speakers to make them work.
A lot of valve people 'end up' with 300B in a single ended configuration, after trying a lot of different valve amps.
300B as a push-pull, well its a very linear valve, so this isnt a bad idea if you need a bit more power but want to keep things clean sounding.
I can't see you not liking a 300B valve amp when its used with the right speakers. Theres little not to like. Similarly, a great valve amp like the EAR V20 or the Graaf GM20 sounds different, but theres an out-and-out quality with top valve amps which make them all easy to live with.
Border Patrol power supplies are the business IMO. Audio Innovations amps and art audio amps are the most common to have the mod. BP power supplies just give you a massive dose of immediacy and dynamics. A very nice thing to have on your power amps.
I think if you buy your valve amp on the used market (and its a house hold name) you will always have the option of changing it later, and thats what I'd personally suggest.
In my case I heard a few pairs of horn speakers and horn speakers with a normal bass cone to supply lower frequencies. I heard things like Tannoy GRF's, Altec studio monitors, that type of thing. What I liked best was a true compression driver horn, with a big ass bass driver. I would have been happy with Altec or Vitavox, but the Altec was the one that came up first. I hadn't heard the Altec Valencia's before I bought them, but had heard enough similar speakers to know what I would be getting.
Moving from Living Voice wasn't so hard in the end. They are excellent and dynamic speakers, but they don't have the qualities of a compression horn, or the bass of a larger cone.