Long before the tube fails completely, it can become progressively weaker. Depending on the design of the linestage, it is possible for the unit to sound reasonably good with weak tubes in it or it can become quite lifeless sounding and noisy. There are VAST differences in how hard any particular unit runs tubes, so one cannot merely count the hours of usage. There were, for example, Counerpoint linestages that would run through a set of tubes in less than a year's time if used every day. The best way to deal with aging tubes is to have multiple extra sets of replacement tubes. Once in a while you should swap in the new, or less used, tubes to see if the sound of the tubes in use have degraded enough for you to want to replace them.
