Then, a few years later, I came into possession of that same pair of LCD-3s, which had not been used in over 6 months by their current owner. The fat bass was back and when compared to my LCD-XCs, was exactly the way I’d remembered it. I literally could not listen to them playing most music.
But after another week of break-in and the two units sounded pretty similar again in the lower registers.
To sum it up: Audeze planar magnetics, at least the models I owned, undoubtedly require break-in in order to be listenable. Furthermore, leaving them unused (at least the LCD-3) for a long time created the need for another break-in period.
Aha!, thanks for sharing @cundare2