As mentioned above, there are 805 / 845 / 211/ GM70 etc big transmitter tubes that are capable to go into the tens of watts range. These big transmitting tubes have remarkably neutral sound, and they mostly take on the character of the driver tube.
So, if you want 300B sound with more watts, go for an amp with the tubes above, that uses the 300B to drive the power tube. You will get the 300B sound but with a lot more driving power.
Still, as above mentioned, at this point you would fare better upgrading the speakers than changing the amps. No power output can replace dynamics that comes from speaker sensitivity.
However, going for more sensitive speakers means BIGGER speakers, so your choice is down to available space.
Cheers;
Janos