K712 BURN-IN comfirmed!
I didn't want to admit it, but when I first got my 712's, the *first* thing I did was run it through a test as kind of a casual thing:
https://www.audiocheck.net/soundtests_headphones.php
So I ran the Quality check - the "Bass Shaker" to see what's up. The first few times, on different gear, straight from computer, or through an amp, at various volume levels, I heard it as it scaled up and down some distortion briefly at various points. Almost as if I had a wire vibrating inside. More of a breathy "pfft" on different notes really.
But those are just test-tones, so I listened for a week, rather than subject them to some sort of burn-in routine. Ok, cool - I'm not hearing that distortion on my music, but aware that it is capable of it. I was especially fearful of tympani rolls, but all was ok.
AFTER A WEEK of normal use, I ran the bass-shaker again. Under various hookups and original settings. CLEAN. No "pfft" - no problems. Whew!
So there - at least I had some sort of non-subjective way of noticing a difference - 'cuz I was initially sweating that maybe I got a bad pair, or sitting in a box for 7 years with no playtime might have been an issue. Wild guess: maybe the varimotion thing just needed some flexing.
Thing is, had I not done this test immediately after purchase, I would have thought AKG burn-in was a total joke. I'm a believer now - at least for this specific model, and perhaps just my specific production. Whatever, the test was reassuring.