I just got some NOS Siemans EL34s......will report later about their sonic character and synergy with Cybers.
The source of these tubes, did some comparison for me and said that they are about 80% performance of famous Mullards.
Amperex 6922s Bugle Boys sounded also very good with Cybers.
I no longer have any use for 5687, they were by-passed with custom board - retrofit and optimized by Bob Beckart, the man behind the mods. Expensive but well worth it.
Every amp that I am auditioning right now faild to surpass Cyber's speed, transparency and dynamics.........the last one was from very well know American Company that costs 10K.
It was simply crushed to pieces.
If you live in NY area and what to compare stock Cyber to modded version, let me know.
Mariusz