In your case you don't need anyone to tell you it's time to change those tubes, or that one is better than the other; you figured it out on your own with your own ears. I don't think NOS is always necessarily better than current production. There's plenty of fine late production tubes out there that sound swell...even better than NOS in some cases, as you've found out. That's not to say there may not be an NOS 6sn7 that bests the EH you like...just that the Brimar's you have don't sound as good to you (they may sound good to someone else though...stranger things have happened). The other way to tell if and when tubes need replacing is to have them tested by someone with a tube tester, or buy one yourself and test them. Obvious signs fo failing tubes are noises like pops and clicks and or rushing sounds. Familiarize yourself with typical tube-life you may expect from the various types you use. I did like the late production Sovtek 2a3 tubes in the Wright 3.5's. I think that's the one tube Sovtek actually does a good job with (haven't liked any of their other tubes I've heard). I hear the NOS RCA's do a good job for 2A3's but you will have to pay some large coin for a good pair. Some company is also making some 300B's that run at 2.5 volts to be a drop-in replacement for a 2A3. You should probably check with George if you want to try that. No experience there. I think swapping any of the tubes has the potential to alter the sound. Probably the least potential would be in the 5y3 rectifier, not to say that it wouldn't have any potential. My experience with those amps tell me that the other two tubes have more impact. YMMV as always.
Marco
Marco