Thanks guys for the input!
To my experience, the break-in hours significantly improve the performance of speakers, and the amps as well. For tubes this is not so evident for me (perhaps, because one usually does not replace all the tubes simultaneously). Though some of you argue that there is a technical reason for tubes to improve their performance with working hours.
The 12AT7WC tubes are from preamp section of Stingray together with a pair of 6414 ones. I bough 6 of each type, new, about 15 years ago or so. It is really surprising that none of them are good now (they are falling within 30 minutes of work). Earlier I had to replace the 6414 ones causing noise problems with E 80 CC (Telefunken) ones, which so far are working fine. Long time before that, I have found an excellent substitution for the power EL 84 tubes with Soviet 6P14P military tubes made in 1986. I have replaced Yugoslavian EL84 tubes with these Soviet ones, which have lasted so long (their metallic "legs" are still of the silver color, unlike the other ones that became almost black). Whereas 6414 tubes caused noise, the sound just disappears and comes then back with 12AT7WC ones (with a khhhh... noise) -- quite bothering, you cannot really audition this way.
There may exist European substitution for 12AT7WC (also not sure if these are ECC 81). I would not like to buy another US made Philips ECS series, but, perhaps, Amperex and Valvo are better.