Electro-Harmonix Fat Bottle 6CA7/El34 vs Sylvania

The Sylvania fat bottle EL34/6CA7 is my tube of choice for my AQ1003DT. However there hard to find and tend to be pricey. The EH fat Bottle is supposedly a duplicate of the Sylvania and I have heard good things about it.

Has anyone compared the SYlvania with EH? How good is it in comparison? Your opinions please.
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.