Audiophiles like to run 300B's as single-ended, class A, which limits
their efficiency seriously. And yes, that way generates boatloads of
2nd-order harmonics. But that isn't inherent in the 300B so much as
it's part of the circuit topology. A 300B can dissipate 30 or 40
watts on it's plate, which is higher than that of an EL34! But single-
ended class A will only give you about 10 Watts that way, max, before
distortion gets seriously annoying. It's not the tube but the circuit
that limits power - despite it's old-timey appearance, a 300B actually
can dissipate more power than some pentodes. If you ran a EL34 in
single-ended class A, you'd get even less power than the 300B would
give.
If you hooked up 300B tubes, a pair of them, in push-pull like a pair
of EL34s or 6L6s usually are run, you could easily get 60 Watts out of
the pair, maybe more. Unfortunately, triodes have a lot less gain
than pentodes, and to get this full output you'd need a driver stage
capable of supplying current to drive the grids positive - class AB2
operation - pretty much like the output stage of the SVT. I think
that this would sound great for a little bass amp. Except for the
cost, fragility, and microphonics of the tubes, which is why I haven't
tried it.