I think you have bad tubes if you even have to ask... Any signs of disinterest in the music is usually one of the signs of tubes on their last legs as they tend to sound flat. I think you can typically expect about 2 - 3 years from a set of tubes in normal listening conditions...
How does sound change when tubes start to fail?
Maybe a silly question, but I'm wondering how I can tell when the output tubes in my Fisher 400 reciever start to die? When I had it refurbished the techs told me the 7868 tubes were nearing the end of their usable life. I feel like the top end is less detailed and dynamic than it had been in the past , HOWEVER, I just moved and have a new listening room with less than optimal speaker placement. Thanks