I have a pair of friends who each has a large stash of vacuum tubes. I will check with them on the availability of the 6LF6 tubes which you require.
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They will glow more brightly,but you will probably notice that they are becoming noisy.When the tube goes,you will measure a DC offset voltage.
Transcendant Sound,Joule Electra,Atmoshere and Einstein also build OTLs.Please check the reputation of the importer for the Einstein gear out of Germany-don't know who it is.
Did you check out tubesandmore.com?
They have NOS for $67 and used for $28.
Also, I wouldn't mix different strength tubes if you can help it. From what I know, even if you bias them correctly, the newer/stronger tubes will burn out more quickly than they normally would if using all of the same strength.
I say if it sound good and isn't ringing or very bright,keep it! Old tube radio's from the 40's many of them have the same tubes in them and I regularly find an old dynaco 70 with original tubes for sale that the sellers says they check out fine,so perhaps just have them checked out by a tech and if they are within specs,hey spend the money on a weeked vacation or whatever.that's my opinion,Nick