Sooo, we have the EL34 which is a nice crunchy tube ........... and it's cousin is the KT77 which is a very nice tube that stays somewhat clean, but gets down and dirty when asked. Does the 6550 hang out with the EL34s or is it more of a 6L6 type of tube? Speaking of the 6L6, it's a nice clean to crunchy USA sounding tube, but I'm not quite sure who is related to it....... .... maybe the KT88? And as far as I know, the 5881 is just the beefed up cousin of the 6L6. So really, I guess it's just the 6550 and KT88 that I'm not quite sure about. Then, let's look at the kids ..... the EL84 is like a mini EL34 and the 6V6 is like a mini 6L6 ? Would that be a fair statement? That's all the tubes I am familar with, for guitar amps anyway. I would be happy if someone could clear this up.