Phantaminum
Headphoneus Supremus
I'd be interested in this question as well since I have Tung-Sol 2C51s with an adapter in my Valhalla 2 (which uses 6922s) and also have a couple of cheap but pretty decent GE 5670s (same tube type). Since I haven't seen any discussion of this, I've just assumed the answer was no. (Though as a practical matter, I think it would be a real PITA to get the adapter seated in the CTH. It wasn't that easy, at least for non-dexterous me, to get it into the Valhalla 2 properly and the single small opening in the CTH looks even more difficult.)
I've rolled 2c51s, 6n3Ps, and 6C8Gs for the past three months and haven't ran into an issue. Now saying that, it is my amp and it doesn't mean your amp won't take a dive. Keep in mind this is a "do at your own risk" and if the amp blows the up there's no warranty.
I will say that 6C8Gs do widen up the sound stage and provides a more holographic sound.