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!