Full Range Drivers and SET's, why is a classic combo?

No crossover phase overlap issues from speaker or amp. 

Point source

low damping factor of SET amp doesn’t overdrive the wide band woofer

SET amp can tame the rising response of a wide band driver in the treble 

no or low negative feedback and no transistor distortion 

wide band drivers can sound dry with solid state amps

most simple amp and speaker type so less to get in the way of the music 

Nice! thanks for your explanation.

What are the main differences in sound between SET's with 300B tubes vs 211?

That depends upon the output transformer (the most important part of any tube amp - PP or SE). And the most costly and difficult to make! It is THE vital link between the output tube's anode (plate) and the speaker. And it's easier to build an OPT primary to match the RP (plate resistance) of the 300B than that of the 211 or similar transmitter tubes.

A 300B uses a plate voltage of about 400. The 211 and similar transmitter tubes use 1200 volts. It is difficult to build an OPT primary that can handle 1200 volts, compared to 400. The same for the power supply components!