How long will tubes last in a pre-amp?

Ask Chris, he'll tell you straight up.

I have yet to see a preamp tube fail, in contrast I've had a few failures in power tubes (KT120, KT88) that were either silent or resulted in a sudden burst of illumination and no more music...

SF Line 1 uses full filament power but a very low B+ so the tubes will be running without cathode stripping.  It is designed to have the tubes warm without being at the expense of tube longevity.  

I ran the tubes in my  CJ PV8 for over a decade with no problem or noise.  However, when I upgraded them, WOW, it was like a whole new amp, a huge improvement.   As the poster above, noted, you might rolling some different tubes through the unit, too.   Different tubes can make subtle to profound improvements in sound.