How does sound change when tubes start to fail?

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!