Lokiiami
100+ Head-Fier
This is a excellent summary!. Wished I knew about this earlier before purchase .Interesting (to me) thought... I am going to guess that burn-in, depending upon the IMR model, will be different depending upon the driver combination as an epiphenomenon of the physics of the different materials that comprise each component and the degree of overlap between them as far as the range they transmit. So, hear me out...
The EST would (conceivably) be the least problematic in that it is likely tuned in such a way as to cover the high end unperturbed. Thus, as temperature and heat generated during use tweak the signature slightly, there won't be much opportunity for it to be out of sync with the other drivers, whatever they may be.
The Planar would be second behind the EST as far as being dependent upon purely mechanical stressors that would affect signal. The larger planar in the later models would also likely carry more load in terms of range spread, and be more likely to overlap, thus increasing the likelihood of harmonic discord/mud.
The Piezo and Dynamic are probably the main offenders. They are the most 'mechanical' in terms of how they produce a signal and would probably change the most in response to temp and heat over time. Perhaps ~150 hours as @holsen has detailed in several posts. They are also most likely to overlap other drivers in low, mid, and highs. Basically, as they shift and other drivers do not, the sound as a whole would hypothetically become muddier in overlap ranges, especially low mids and upper mids. Here, they would undercut the planar magnetics for a time.
As Bob is clearly a master tuner, I would guess that he would aim at a couple hundred hours as a place to sync the assorted drivers--whatever they may be. That would be logical, given that all drivers are likely to settle into their long term formations by then. What that would also do, however, is to cause a period--or several--wherein drivers would be out of sync with one another. These might be the "70 hours" or "50 hours" wherein maximum distortion is present, and people send 'em back...
I dunno. What do you want? I'm a scientist, LOL.
And I noticed IMR doesn't use BAs for their drivers ?