Tubeaholics please advise

I have the Counterpoint SA-4 OTL monoblock amps, which I bought retubed about 10 years ago, running Martin Logan CLS's. No problems so far, but the mileage is getting up there. You cannot bias the SA-4 tubes individually, so matched sets of tubes are recommended. They take 8 6LF6's per side, which I am now finding out are nearly impossible to find NOS in any quantity, and if you can find them, are going $100.+ per tube!
Issue: (1) should I buy a few used tubes and keep nursing the amps along, despite the fact the used tubes will not be matched? Or just start saving now for a new set of amps? (2) given that OTL amps can fry speakers when the tubes die, what are the telltale signs that a tube is about to go?
thanks guys!
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.
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.
If you love those amps with your CLSs (and you should!) have this guy upgrade them to the max, which includes a separate biasing unit that allows the use of unmatched 6LF6's:
The offers I posted on, use the 6as7,6c33c-b and the EL-509/519s.The last are probably the hardest to find,but I understand that Transcendant carries the tubes for his amplifiers.
Did you check out
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.
The EL509 is probably your best bet if you plan to update the amp for a new output tube, given the scarcity of the 6LF6s.

If I were you I would contact either Altavista or Rodger Modjeski, who put in a lot into the design of this amp.

Good Luck!
I have such large stash of 6LF6 GE NOS tubes that Roger Modjeski graded and matched for my Counterpoint SA-4 amp. Back in 1988, I found out the tubes were being discontinued so I bought a stash of 250 tubes.
Check out Easy to work with. Good luck.
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