No reason to replace the GZ-34 unless it goes bad and that may take decades.Not a fan of 7199's - they don't age all that gracefully - might want to consider some of the alternative driver boards - nothing actually just substitutes for the 7199 - weirdly enough - I'd consider replacing these first -simply because in a couple of hundred hours they don't sound all that good anymore.When I owned and serviced 70's - had a strong preference for EL-34 Telefunkens -am a little shocked what purported NOS's of these go for nowadays.
