Leben CS600 required tubes

I tried most, liked the KT66 but the EL34 won out. 

Another vote for KT66 though I've not heard the 350B. I also like the KT77 by Gold Lion not personally a fan of KT-88 or higher.

I would suggest Tesla NOS EL34s as another option.

I agree regarding the 6P3SE; a bit too forward.

Of the 6L6 world, I recommend GE NOS

I've got Mullard EL34's (produced in Russia by New Sensor) in mine now and they sound great. I didn't like the Russian NOS 6P3S-E (I only tried one plate voltage...) or the stock 6L6GC Sovtek's (altho I liked them better'n then the 6P3S-E's).

Thinking about trying the Genalex Gold Lion KT-88 reissues next....

KT66 IMO. The 6L6GC is also relaxed but isn't as resolving as the KT66. I currently use Groove Tubes KT66C with a hardness rating of 10 in my McIntosh MC240, and I find them to be excellent.

Arthur