Is there any 50W 300B SET monoblock out there?

KR Audio produces a 50 watt SET, but not using a 300b (of course)

Why would they want to?

The tubes used in the application of a SET design are the limiting factor not the engineering. There is only so much power a 2A3, 300B, 845 triode can produce. What you need is a tube manufacture to manufacture a more powerful triode tube.

As Brf indicated, there is only so much power a given tube type can produce. Using two or more power tubes in parallel, in a single-ended configuration, would allow more power to be generated, and is sometimes done. That would be referred to as a PSET (parallel SET) configuration. Alternatively, the VAC Renaissance 70/70 MkIII amplifier I have uses four 300B's per channel, configured as two push-pull pairs in parallel with each other. It is rated at 65 watts per channel. But of course it is not a SET (single-ended triode), because it is push-pull rather than single-ended.

Regards,
-- Al

Check out the Emission Labs website. They have several 300b variants that can put out around 20 watts.