Favorite 300B: Sophia vs KR

I have heard different stories that tubes can still test strong after many hours (5000 plus) but loose their life in sound. I think I have discovered this with a pair of 45 tubes that were driven "softly" by a Korneff amp. yet sounded dull (on horns) after 2 years and logging many hours. A quick check in the Hickock 600A showed a lot of life but a change to some similar tested pair brought back the life in the music. The tubes with a lot of hours still test swell today but sound soft. Not only this but some designers will tell you the same...so I think you have to trust your listening skills and have another pair to compare.
In my heart I have to believe if you get 5000-8000 hours on output tubes you might be doing good.....40,000 hours is non-sense IMO. I am glad that Phaelon mentioned this because I had no where near 150 hours on my RP tubes...maybe they are asleep yet?
My little fan idea gave me a piece of mind that it is the best I can do to extend the life by trying to cool the tubes. Once I connected the variable voltage cord and turn to ultra slow speed I cannot detect the fan running at 2 feet away but the tubes run much cooler...on the outside at least.
There are so many experiments going on in my man-cave I cannot keep up with them all.

Not sure that I understand how "tubes can still test strong after many hours (5000 plus) but loose their life in sound. .. "

From my own experience I know only 2 types of tubes that sounded freaking good, while they tested poor.

1 NOS Tung Sol 6550 Black Smooth plates

2 NOS GEC KT88

I owned quads from both at the same time and in each quad was 1 tube that tested poor, but they sounded great. How possible? Don’t know, it's all beyond me, probably the quality is so high that they still perform until they really die.

THX

Gwalt don`t know that the 40,000 hour claim by AVVT and W.E. is true or not. What are you really buying with these 'old stock' tubes that still test well? you won`t ever really know their true history of use. It`s a risk some are willing to take at ultra high prices(W.E.). I`d rather go with a new RP or Takatsuki with a warranty but of course to each their own.
Maxmad, you raise an interesting point. There are 'some' tubes that test weak yet sound good.Tubes can be a fasncinting mystery.

Charles1dad I agree with my NOS 45's you never know the history and only test how high the microbos/ma rating goes on your tester and if they are quiet with no shorts etc. I am only relating my experience with 45's (not 300b tubes) purchased on EBAY @ $100-$150 per pair on a simple made tube in a simple circuit from sellers with stellar feedback and experience with tubes. I had put a lot of hours on the ones that sounded soft in the end and got my $100 out of them. Current production 45's I tried were quickly sold off in favor of NOS.
I think you have a lot more to risk in the NOS 300b tube with WE prices (not for me) and lack of any warranty and totally agree on that. For me I guess I would like to hear more from someone who has the extra long 300b tube life chime in with their experiences. I had two long time manufacturers tell me the same thing. Anyone out there with over 10,000 hours on a 300b tube that would care to share? I would love to know what to expect for the price we are paying for the tubes.