There's a difference in the cautionary warning note between the amp and preamp. CJ states in their manuals that they have selected the best tube possible eg. of what's available and then they burn them and select the best of the best using their in circuit performance (that arguably is the best way to do it). One of the things that cj obviously selects for is tube quietness-as one can hear with their selected phono tubes.
Now with the amp, I was talking about the possibility of damage, not as in the case of the preamp with degradation of the piece's sound. Now are the cj tubes the best sounding tubes? That can be argued but they are probably the most reliable tubes that one can use. And all one needs to do is go back a couple of years and hear how many audiophiles were turned off to buying tube equipment because of relability issues.
That said I have tried different tubes in my cj TEA 1bc phono section. I've compared probably a half dozen or more brands in the unit (Mullard 10m, Tele rib plate, Amperex, Mullard early '50s, Siemens dual post, etc) and ended up using the Tele rib plates. Now the tele rib plates are not as quiet as the stock tubes--and that may be an issue for those using LO MCs--but they sound much better.
Now one might be inclined to go to the several tube sources that sell selected tubes -- but how long will these tubes stay quiet and there's the question of cost. Yes one can use some Tele CCAs in the ART preamp--but at the cost of what $300 or more times 10 tubes, we're talking about a small fortune, even if one says these tubes will last 10,000 hrs (and they may last, but how long will the sound good for?).
Now could different 6922s sound better than the stock tubes in my GAT? I don't know but someday will try some of the tubes have accumulated.