I think part of the sound difference maybe due to the transformers used. I saw this in a posting about transformers at Tales from the Tone Lounge. Also if the amplifier was designed with feedback as most Fender amps are (6V6 6L6). I think most English amplifiers (EL34 EL84) did not use feedback and used over designed (big) output transformers. While Fender amplifiers look to always have feedback and somewhat smaller transformers. The feedback would actually allow use of a smaller transformer by reducing output impedance so lower reflected impedance needing less inductance for bass response. But if you are going to drive the amplifier into distortion then feedback will make the transition into over drive abrupt causing another tonal difference between the amplifiers. So it may not be a simple question of the tube in the amplifier but the design of the output stage of the amplifier and the transformer used.