I have owned my current Tube Research Amps (mono 100's) for 3 1/2 years without failure, nor need for tube replacement. (I've checked on TV/7 and Hickok 752A tube testers). Before this, I had Tube Research 300 Mono's for 6 years, once again, no failure, no need for replacement. I, like Kirk 930, run mine 24/7. And, they work well with my Avalon Ascents, Sound Lab, and JM Lab.