when i first starting posting here, there were several guys who had them, they were kinda popular here then...i don't hear much about them now though?!
My former mig 50 is kind of a dark amp that seems to love humbuckers. It doesn't like singles all that much. Not a bad amp as it seems to love pedals and is pretty flexible with the right guitar.
You mean 6L6s or 5881s, which are the typical Fender tubes, and were used in the Sovteks.
Marshall actually did use 6550s in some of their US amps, 6550s are like really hard, clean EL34s (for comparison only, their bias is different and it usually requires amp mods to go between 6550s and EL34s).