What I am describing I don't think is the linearity changing with increasing volume but just that of a boosted upper range to begin with (see frequency response plots at headroom). Waterfall plots of the MS1 also show some stored energy going on in this region and by looking at the Grado, resonances showing up in the upper midrange on the impedance graph.
It just becomes noticeable once you start to increase volumes. At low volumes everything sounds great, cymbals especially sound better than the AT AD700 at low volumes. They have a lot of sparkle to them and in comparison the AD700 sounds dull. The only time I can listen to them at low volumes is if the noise floor is really low in the house because of how "open" they are.
I can tell you that the AT AD700 is one of the most comfortable headphones out there but they are big and ugly (purple grilles and gold frame). They can present a huge soundstage especially if you are into classical recordings.
The AKG 271 MKII is the most comfortable closed headphone I have worn. No clamping at all and I can wear them for hours without any comfort issues. This is in comparison to the Shure 440, Shure 840, Sony MDR V6, AT M50.
Both the AD700 and MKII will disappoint if you are a basshead or into thumping bassy music.
$200 is not a bad price for the MKII in Australia. Keep in mind it comes with two different pad options (leatherette and velour), two detachable cords (straight, coiled mini-xlr, 1/4" adapter)