Tone is very subjective Mark2C and I don't want to lose that EL34 mids, especially in an Einstein which is a very heavy mid amp. I would ask you this, how many world class guitar amps use 6550's for their power stage? Not many I'm aware of and I've been a semi pro musician for almost 25 years. It's usually EL34 or 6L6 in all of the coveted classics. Seems to me even Herr Diezel is chooosing this tube due to poor performance and reliability issues of the other choices, not because he loves the sound otherwise it would have been the stock tube the amp ships with in the first place and his first choice as a replacement, not his third. Most amps and their transformers are also optimized for a particular tube type and while others can be biased to work, how well they work is another story. This amp was designed around EL34's and their tone. My understanding of 6550's are they are very even as you stated but carry an almost solid state wide band tone to them, that's why they are used in many high end tube power amps for home audio. I could see someone playing metal thru a Herbert liking this but I like the british/german elements of my Einstein very much and love the more old school traditional sound so that's why at least for now I'm going to stick to EL34's. Thank you for all of your efforts to explain the 6550 (I mispoke when I said no one could tell me the change to expect but really no one here seems to have done this w/an Einstein either and no one spoke to the critical mid range or lack of snarl (for lack of a better word) that a EL34 provides in spades. I'm sure I will eventually try some 6550's and if by chance someone else does put them in an Einstein please report your specific findings. Thanks to all.
