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?
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 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.