Electro-Harmonix Fat Bottle 6CA7/El34 vs Sylvania

From my experience, EH tubes are going unstable very fast and may cause damage to your equipment.
Yugoslavian 6CA7 fat bottles may be your budget key instead of Sylvanias. They're very stable and reliable. They produce unbeliavably clear trebbles and mids. In well-designed tube amp you would see almost no compromise in comparison

How would Yugoslavian 6CA7 fat bottles compare to Svetlana EL34's in a Conrad Johnson MV-55?

I used Svetlanas in my VTLs and now use Yugoslavian ones.
They have more presence and details i.e. I assume less distortions...
I can't tell about CJ since an output transformer in tube amp is another large variable to be in synergy with tubes.
Despite EH having larger bass than both of Svets and Yugos I would never use them in my amp again due to the bad reliability.