Freediver ...Since the sound of the tubes is fundamentally different between the 300B and KT150 tubes, the rest of your circuit would need (as they say in the fine watch world) "complications". Why not avoid adding what isn't there and simply embrace building a 300B SET amp? There are several excellent kits including ones from Audio Note Kits and Bottlehead. ...Or, you might find a completed kit or commercial 300B amp here in the classifieds.
I'm currently listening to three different tube amps: Rogue Cronus (KT120), Ayon Audio (KT88) and Raven Audio (6L6). They could not sound more different, and in the case of the Raven (6L6) and Ayon (KT88) amps, it's the tubes. The Rogue is a very good amp also, but at that price point there simply isn't the level of components due to cost burden. I have listened to 300B amps and they do sound glorious for some genres of music. (Eventually a 300B amp will appear from a UPS package as part of my personal education.)