People ask crazy money for engraved-base 300b’s. I saw seversl asks of $20k for a pair. Of course, ask and whst they get can be WAY different. I heard several push pull and a SET running those tubes (two pairs per channel).
As crazy as are those prices, 300a’s are even more rare and expensive. I’ve only seen and touched a pair, I did not hear them. The dealer and builder of custom amps was collecting them for a customer who need two pairs.
If you really don’t know much about these things, sell them. Do not use them in any crappy gear; they deserve to be used in really good amps.