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

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!

The SE 300B can do about 7 watts at 1+ per cent distortion. The 211 and similar transmitter tubes can do about 15 watts SE. That is about a 3 db difference in drive capability for a whole lot more cost and complexity!

A good single-driver speaker for low-watt SE amps is an Altec 755. I had a pair. Put it in a one-cubic foot box and it will work from 60 hz to 12k hz with fairly flat response and enough sensitivity! Add a subwoofer for the bottom two octaves.