Output Tube Question

[54strat]

I've been running some JJ 6CA7's for two and a half years. As explained in another thread, I have loved these tubes and wanted to see how far they would go. Bias has not drifted and they have sounded great until today. While fiddling around at home, I noticed a sudden reduction in volume after playing a couple of minutes, not major but noticeable. The amp didn't sound quite right. I put my spare pair in and noticed the new tubes had more beef and more volume. Put the old pair back in and really noticed a loss of beef. The amp did not sound completely awful just weaker and missing some of that chunky beef we all love.

Here's where it gets weird to me...put the bias probes and old tubes back in to check them out and one of the tubes would not get any bias reading AT ALL. The other was about normal. Switched the tubes around, switched the probes around, the no bias reading always followed the one tube. I simply have not heard about this sort of thing happening, where a power tube will not get any kind of bias reading at all, yet still work at a reduced level. Is this "normal" for a tube about to die? Did I happen to catch it before it blew completely out?

Both new tubes bias up fine, no other issues apparent, and the amp sounds great again.