Thank you for the good recommendation. I received the Genalex Gold Lion Cryo 12AT7's (matching pair) and replace the stock tubes in my FET Valve Hybrid 600R amp. I let the system warm up about 30 minutes before I played anything. Then I started listened to digital music (FET Valve Hybrid DAC) and all I can say is WOW!!!!!!!! Compared to the stock tubes these Genalex are a GREAT improvement. There is a wider soundstage, cleaner, more colorful, more resolute, and sweeter sound that is more life like - realistic. The tonal quality is truly a magnitude better than stock. Horns, piano, drums, symbols, voices sound even more life like than before. I only have a few hours of runtime now and having a hard time turning it off, so far so good.
When I first turned the amp on was worried because these Genalex tubes don't glow and I thought something was wrong. So I double check all switches and connections and then tried some source music and the WOW began.
If you have not heard your FET Valve Hybrid 600R with these tubes then you really should treat yourself. I can definitely recommend them for this amp.