Yep, I tried them in the Mod 50. I thought the Svet 6550s might be stiff sounding or hi-fi so I went back to EL34s. Sometimes the mind/ears, plays tricks, LOL! I re-installed the 6550s and the amp sounds nice and fat, not sterile or stiff at all. Still growls.
I tried the 100 watter, but prefer the Mod50, I find it fatter and warmer - more my thing. It's not like I play stadiums without a PA! LOL. I couldn't justify owning a 100 watt head.....I even feel weird having a 50 watter. Heck, I know I could get by with a small combo with a good PA.