Mcentee, the last tweed Pro I owned was a 5B5 that I resurrected. It sounded fantastic.....and was dissipating a bit over 52%. I found that perhaps a bit odd, but there was arguing with the sonics. Perhaps Leo was not looking for all of the heat that could be had????? Anyway, it had great frequency response. Great cleans with articulate lows. When pushed, it had very good breakup. I never gave a thought about lowering that resistance in the bias circuit because I simply could not justify changing the sound. What would a different set of tubes have done? It would have been interesting to have had some HOT 6L6’s to try in the amp......for curiosity’s sake, I suppose. But again.....I was not driven to change anything about the amp. The Pro Sonic uses a 220ohm for the cathode biased option, and I have measured 100% plate dissipation in them. GZ34/5AR4....?????second half of the 1950’s is the first time I have seen them. The first 5F6 Bassman use the 83 rectifier. The second version used the GZ34....1957 or so, iirc. Leo Fender used two 5U4’s in the low power 5E8A Twin to make the amp stouter/tighter. If you pull one of them, that amp becomes something more like the 5E5A Pro/5E7 Bandmaster/5F4 Super Amps.