I have a 300B amp in atm and to be frank it wasn't anything to write home about.
Very well proportioned output transformers but lack lustre bass and a thickening of the sound as the wick was turned up.



Investigation revealed it to be a zero feedback design, now IMHO that exposes all the weaknesses of the active devices, not an approach I am keen on.
However with a small modification it transforms into a stellar performer. Gorgeous Class A clarity and with a really solid taut bass that you would hope for from generous output transformers.

I've not heard a 300B amp before but this one now is world class, sensational sound.