My present custom build 300B SET monoblocks are my all time favorite amps. I've been though class A/B, class A SS, push pull tubes with some of KT family, 6550, EL34, 2A4, 845 and 300B SET. What you're hearing is what I call immediacy, SET has performer in room transparency, relatively few parts in these low power tube amps very close to straight wire, signal doesn't have to go through myriad of parts, circuit boards. With top flight power supplies and sympathetic speaker partnering these amps capable of playing any music with authority, I certainly don't avoid any genre of music.
As for 300B tubes, all the top flight have their fans, can't really go wrong with Takatsuki, EML, WE, Elrog, Psvane Acme, Audio Note, probably forgetting others. These all play at much higher level than rest, meaning higher resolving, transparency with these. I do see occasional failures with 300B, but nothing concerning assuming amp doesn't overdo it with plate voltages