Hi Pilot 1.
Yes, you're right, Guitar does not *need*much above 6 or 8 KHz, which wonīt be reproduced by the speaker anyway, but bandwidth does not hurt either.
Any popular Power Chip , from TDA2003 to TDA7294 easily meets 50KHz or more bandwidth, so no intrinsic problem with that.
The schematic posted above follows the very clever Music Man ideas (search http://www.music-man.com/techinfo/old-amps.html
As an example:www.music-man.com/techinfo_old/old_amps/2165-rd_&_2100-rd.pdf
They could achieve incredible feats:
1) slamming brutal 700V on plates (650V on 6L6) without burning them; in fact theyīre known for their reliability.
2) Pulling 65 REAL watts from a pair of tubes, up tp 130W from a quad.
3) no tube biasing or matching necessary, the SS side of the circuit takes care of that.
4) they could drive those tubes fully with a simple dual Op Amp and a couple mid power transistors
5) Loud, clean sound, way beyond what, say, a Twin could provide (the reference amp up to that point).
6) not surprisingly, it was a "real" Leo Fender product and concept ... although he couldnīt say so for legal reasons.
Leo, you are my man !!