Thanks for the (almost?) free advice!
I would have thought there would have been some noticeable difference in sound, so was surprised that the half-heated tube was managing to keep up. Makes sense it would have a much shorter life though...
I got this lot a while back from an ebay gamble - no real description, and they appear to have seen some good use... NOS prices for these tubes are too crazy, not worth it IMO! I went through the rest of the lot and happened to find a 2nd TFK 6080 that also has only a single heater powering up, as well as one with both heaters working. Coincidence multiple tubes with dead heaters in such a small sample? Perhaps a common failure with these tubes? Either way, I've set the "half-heated" tubes aside as a sad pair that can limp along together, hand-in-hand towards their inevitable demise.