Preamp tubes are not too stressed, they can last a very long time, except in this one I know of, Audible Illusions 3A they ran them very hard.
https://www.google.com.au/search?q=Audible+Illusions+3a+preamp&espv=2&biw=1680&bih=944&source=lnms&sa=X&ved=0ahUKEwiAvLuw85XQAhWEpZQKHYh8Bc4Q_AUIBSgA&dpr=1#q=Audible+Illusions+3a+eats+tubes
Check for micro-phonics though by gently flicking the top of each tube with the finger nail while at normal listening level, have some one listen at the speakers, very small noise is normal, loud ones can mean this tube has gone miro-phonic.
https://www.google.com.au/search?q=Audible+Illusions+3a+preamp&espv=2&biw=1680&bih=944&source=lnms&sa=X&ved=0ahUKEwiAvLuw85XQAhWEpZQKHYh8Bc4Q_AUIBSgA&dpr=1#q=Audible+Illusions+3a+eats+tubes
Check for micro-phonics though by gently flicking the top of each tube with the finger nail while at normal listening level, have some one listen at the speakers, very small noise is normal, loud ones can mean this tube has gone miro-phonic.