The 45 is one of the best sounding SET tubes. It is partially because the tube is very linear and possibly because it is much easier to build wide-bandwidth output transformers for low-power SET.
A 300B - almost any - with some somewhat thicker, slower, and less extended than a good 45. And the Korneff amp is one of the very best. I have not heard better. I prefer it ultimately to the Yamamoto.
This may not be quite true for extremely expensive 300B SET amps. Certainly you will need to spend much more money to get the transparency, speed, and extension the 45 will give you.
However, while a 45 will play your speakers quite well, at loud volumes it will suffer dynamically in comparison.
Still, I'd go with the 45. I've had K-horns and Fortes.