Too bad!!
Interesting, though. I asked about this same thing a couple months ago, but nobody seemed to agree* at the time, so I chalked it up to my amp or my ears. In fact, I even "got used to it" to some degree, but every now and again I'll have them at lower volume and find myself thinking they sound veiled or muddy, or something. I'd have definitely mentioned this if it was something I thought was worth mentioning.
I wonder what it is that makes them more suitable for loud(er) listening? I'd have thought it was the drivers, but if the Amiron are better at lower level, maybe it's the tuning? My PM-3 are definitely my most laid back headphone and also the best for playing at low volume. I always thought it was because they're planar, but maybe it's because of the tuning. Huh. Food for thought.
Also, if returns are on your list of priorities you should probably skip MassDrop, unless they've changed their policy.
*Or, well, now that I'm thinking about it, maybe somebody did agree, but I preferred to hear/remember only what I wanted. Such is the mind!
They are tuned different, the drivers while essentially the same driver are also tuned differently. I swapped the Amiron Home and DT 1990 drivers in the past a few times and the Amiron drivers kept some of the Amirons traits such as it's ability to play well at lower volumes while the DT 1990 kept it's traits of playing louder well and it's more intimate sound.