Do tubes become mismatched as they age unevenly ? When they were almost perfectly matched.

I’m no expert but my belief is they can and do. Good question. I wouldn't claim to be able to hear it however.

I think, one pair of Mullard EL34 XF2 output tubes in my VAC Avatar SE amplifier became a little mismatched judging by slightly different positions of biasing knob. When I got these tubes used but test NOS more than a year ago it was symmetrical. But after 1500 hours this changed. I don't hear the difference yet. Should I rotate the tubes, I wonder ? Another channel is the same as it was. I bought a matched quad. Don't have a tubes tester so can't be fully sure what exactly I bought, though, but the seller sounded very honest and knowledgeable.

Can you hear it ? A little more noise and distortion ?

@inna  Yes. No tube or transistor is perfectly matched. Most 'matched' power tubes are only matched at one operating point, usually on a tube tester. To really match them properly you have to use a device called a curve tracer, which can display the linearity curves that result as the tube is subjected to a variety of operating points. The first is a 'static' match and the latter a 'dynamic' match. 

And then the tubes get installed and over time they drift. Its rare that a pair goes downhill and stay matched. So distortion goes up, output power goes down and output impedance goes up. You can hear it!

Ralph, I see. Yes, I heard about a curve tracer. Brent from Audio Tubes used to offer this dynamic match service for a small fee.

But I got it. If I don't hear the deterioration I am fine.

Happens all the time.