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