Well, the 300B has been around a long time, and is well-known to be a very good sounding and very linear tube. Part of the "reverance" about this tube belongs to the SET circuit designs in which it is usually found. SET amps are particulary coherent sounding, due to certain design characteristics of that configuration. You are now finding that some people prefer 2A3 or 45 over the 300B in SET amps. One of the "revered" characteristics of the 300B in SET amps, is the 300B seems to have a lot of "meat on the bones", meaning it is somewhat fat sounding and has a healthy bottom end.
Regarding your question about the "high frequency performance", the 300B and other triodes have great extension into the high end. The problem arises in the output transformers that are used in these SET amps. If the output transformer is not sufficiently good, the high-end will be rolled-off. This is not a problem with the tube, but with the output transformers. If you get a 300B amp with really good output transformers(which are expensive) and has a good circuit design overall, then the frequency extension is fine.
Regarding your question about the "high frequency performance", the 300B and other triodes have great extension into the high end. The problem arises in the output transformers that are used in these SET amps. If the output transformer is not sufficiently good, the high-end will be rolled-off. This is not a problem with the tube, but with the output transformers. If you get a 300B amp with really good output transformers(which are expensive) and has a good circuit design overall, then the frequency extension is fine.