Call Conrad Johnson. In my ARC LS 16 I called them and they said it was no problem. My favorite tube is a 6dj8. It's only a problem if the tubes are being driven hard. Maybe ask this question over at AA also.
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I know it's a bit late to post this response, but to answer your question, you should have no problem by sustituting 6DJ8's for the 6992. I did it on my Premier 16LS and it works just fine. I found the 6DJ8 to be a much sweeter tube, not as bright as the 6922. I heard more midrange resolution with it, more detail and definition. From what I can remember, the 6922 is a military grade version of the 6DJ8, they essentially are the same tube with different designations.
All work fine in Premier 16s and 17s. You can mix brands, but cannot mix types (ie., must be all 6DJ8, all 6922, etc.)
I agree Sanji, 6DJ8s sound sweetest, I like Amperex 6DJ8. 7308 is long life version (~10k hours). 7308 has a teeny bit more top end extension and better low-end articulation in my system, but not quite as sweet in the mids. Telefunkens are nice too; I could never prefer more than 1 Siemens 7308 per channel.