Heater filament glow is not a good way to assess tube health. Unless there isn't any glow at all. Preamp tubes are not beaten up the way power tubes are, so they should last for many, many, years. I'm running 30+ year old tubes in some of my amps, probably with 1000s of hours on 'em, and no probs.

12A_7A and ECC__ are interchangeable. The only difference is the tonal characteristics, and that's a subtle thing, subject to a lot of cork-sniffin' tonemeister disputation. Feel free to swap things around. You won't damage anything, and you may just stumble onto a magic combination.