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
My experience is with 300B tubes in my VAC Renaissance 70/70. When tubes started to “give up” I have heard the following: decrease in dynamics, lowered high and low frequency output, “muddy” sound. After re-tubing I was amazed by what’s been missing.
I hope this helps…
Hi Daka. Meta's description is a good one. Another tell tale sign of a spent 7868...... look at the silver flashing[gettering] on the top of the tube. It should look as solid silver down below the shoulder of the dommed top. If you can see the getter or there is very little silver flashing left, the tube is just about spent or should I say tired. As to your techies comment: I am certain that your techie must have noticed that there was very little getter flashing left to make a statement like that. I have seen original Fisher labelled sylvania 7868's with almost no flashing left but still test quite strong. Your Fisher 400 is truely one of the great ones and with a top shelf tuner to boot. Perhaps it's time to retube it. Best of luck!
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...
If your system sounds a bit tired and lackluster, it's either you or the tubes have started to go bad. Sometimes older tubes develop noise problems as well (sputtering and whistles, etc.).
Think of it this way, one of these days you'll have to retube the thing. Might as well get a set of new tubes now and try them out. If you hear a difference, well, then it was time. If you hear no difference at all, then put the old ones back in and rest assured that your time will come.

Very good advice. However, I am always trying different tubes so its hard to say whether the difference is attributable to wear or to the inherent difference between the tubes.
Thanks so much for your responses. I hope my system sounding a little lackluster is not me! I hope its the tubes or new listening environment. Mezmo, your advice makes sense to me. Does anyone happen to have any experience with the new EH 7868 which I just saw advertised on the TubeDepot web site? Would I be 'better off' with used or NOS or try the new EH? Thanks again.
I have yet to hear an EH tube I like. NOS! NOS! NOS!
Hi Daka.I really have to agree with Boa2 on the nos tube issue,especially in the 400. A good friend purchased a clean refurbished 400 with tired output tubes a few years back and then retubed it with the EH 7868's.He uses a nice pair of vintage rogers ls3/5a's. He couldn't understand why my 400 sounded so different with the same speaker in my office rig.I insisted it was his EH tubes and he insisted that his wife would not spend the extra cash after buying it, especially when he could retube it for around $50.00. I visited him one evening with my quad of nos sylvania's in my pocket and both he and his wife nearly fell off the couch! Best of it is... his wife was really pissed that he wasted $50 bucks on the EH tubes. His eh 7868's have been relegated to tube drawer. There is really no contest here sir. You will get 2500-3000 hours of beautiful music out of a quad of nos 7868's. You and your family have a great Christmas and all the Best in the New Year. PS: sounds like a great stocking stuffer!