I replace tubes well before the getter "flashing" gets funky, and often simply because I want to try a different tube...generally the power tubes get the most attention, but my amp has only 2 (single ended pentode) so it's cheap. Preamp tubes seem to last a very long time in my Freya (4 6SN7GTYBs), and there's only one of those in the amp that also lasts...I check tubes by simply replacing one from my pile of new spares.
