The original S.E.X. amp, from the mid-nineties, popularized the 6DN7 enough that the supply seemed to be getting tight by the time it was replaced (by the original Paramour). I think a bunch of people started to hoard the tubes; there were by then several 6DN7 amp circuits making the rounds of the DIYers.
The stereo version showed up a decade later, and it was deliberately designed so that it could be converted to the 6EM7 which has several variants. The same is true of the new version, though we have de-emphasized that because, as it turns out, we have not had much trouble finding tubes. It's a TV tube and was made by the millions back in the day - perhaps the tube dealers started to save them out instead of junking them as useless? In any case, these old TV tubes are pretty rugged and reliable.
The only exact substitute is the 6FJ7, which has a Compactron 12-pin base.