Kirk at Cary recommends GE or RCA 5691 Red Base NOS tubes as a replacement for 6SL7GT's. Cary sells the 5691's. Check their website at http://www.caryaudio.com/accessories.html.
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You should try Svetlana or JJ EL34 tubes. They both will give you a nice improvement in the sound. Which you like depends on your tastes.
The 5691 recommdation seems like good advice. Especially, coming from Cary.
This amp is a little underpowered in my opinion. If you ever come to the same conclusion, you could give JJ KT88 tubes a try. I was once interested in this amp, but found it not up to driving the speakers the seller paired them with during my audition. I called Cary about using the larger tubes, and they let me know I could. From what I remember, the amp may be autobias.