Marshall's were sold for a period of time with 6550's in the U.S. market reportedly due to fragility of EL34's at the time. The current required by the two tubes differed and therefore 6550 equipped amps used a 47k bias resistor and 150k Grid leak resistors while EL34's 56k and 220k's with a few exceptions. Both versions used .022uf PI coupling caps. Grid leak resistors and PI coupling caps form a high-pass filter and halving the resistor value raises -3dB corner point one octave. No doubt amplifiers fitted with 6550's have a different vibe than the EL34 models each with their own fans. Placing the bias /current differences aside it would seem that a change in coupling cap values would have an impact on tone/sound including bass response. KT66 equipped Marshalls used 220k grid leaks with .1uf PI coupling caps as an example. Also, my understanding is that reducing grid leak values will load the PI differently rotating the AC load line clockwise changing its clipping characteristics. It makes sense to me that Marshall would change as few components as possible to get the 6550's to function. As the amps in this era were ST1 PC board versions it would have made no sense to change PI coupling caps to a more "suitible" value for the 6550. I also think that Marshall felt that amp buyers would convert back to EL34's and most did, not all in the best manner. Has anyone played with different PI coupling cap/Grid leak resistor values with EL34/6550 equipped amps and what were your take aways? There are calculators and I have fooled with them a bit but they don't really offer any real world tone findings.