Tube life expectancy: Stax driver unit SRM-006tII


Hi, does anyone have experience with the origin and life expectancy of the 6FQ7/6CG7 in the Stax earspeaker driver unit SRM-006tII? Since Stax are very strict about this, I haven't opened up the case, so I don't know what make they use. Since I use the earspeakers not regularly, I don't leave them powered on when not in use. Any rolling findings with the unit?
Thanks
karelfd
Tube life in these units last 5000 hours and up.
Mazda(Great Britan)6CG7 from Upscale Audio at $120 a pair might be a great candidate for rolling.
Thanks, in particular for the rolling recommendation since I haven't found a great deal of information on this type so far;
Fwiw some folks on diyaudio.com prefer the 6FQ7 over the 6CG7. Any comments on the differences?
I use the Mazda's from Upscale. Good News is that they are only $80 per pair not $120.