Re: EL34 vs 6550's
Nearly all European Marshalls have EL34. (yeah, some come with EL84 and I am sure I will be panned about Anniversary models which have 6550's) I am generalising here.
In the past a distributor in the USA demanded that Marshall supply 6550 valves for the US market as there were complaints about EL34s breaking in side the warranty time. That is the reason why US players wondered why their British counterparts sounded different. I would stick to EL34 as it is part of the charisma of a British sounding amp. Kerrang!!