Tube rolling on Leven CS 600

For an EL34 alternative try the Gold Lion KT77's a great sounding EL34 variant. Also NOS 6L6 tubes are pretty affordable and might warm things up over current iterations. Also oldhvymec is right the small signal tubes have a large impact on the sound. That's a great little integrated have fun tube rolling!

a great 6L6 family tube is the Tung Sol 7581A....my experience with the Gold Lion KT77 is terrific tone, detail and soundstaging but deficient in the mid and upper bass. The 7581A has stellar bass while being a very transparent bottle.

Has anyone tried the Gold Lion KT66 in the Leben CS 600?

I've used the GL KT66 in this amp. They soften things a little but also seem to have slightly less in the way of focus with the O/93. It depends on what you are after.

Interesting. 
I had the 600 with Devore O/93s. After running various EL34s, GL KT77, KT90, 5881, and the Russian equivalents Jeff Day popularized, I eventually settled on GL KT66, which I found to be the most balanced.
I'm usually not one to put much weight on the notion that cables can transform a system, but: which cables are you using? I find that the apes like being fed with A23 wire.
All that said: the Leben is not a "tubey" amp and in my experience no amount of tube rolling changed that. (Nor experiments with footers, dampers, and incantations.) If your streamer follows the traditional Linn house sound, then it majors on fast and "clean"; I've also found the Feikerts I have (briefly) heard to be somewhat in that direction. Probably not what you want to hear, but them's my 2 cents.