It's not really a stretch to say so. My comparison between the two proves that very fact to me anyways. The Denon might have more lower treble presence, but I still get a feeling of a decent treble spike in the upper treble range on the HE-400. I just compared the two again for reference using Normalize by Flux Pavilion and Stayin' Alive.
I wouldn't call the Denon D2000 too extreme. It's edgy yes, but nothing's extreme once you've heard a DT990. Soft and resolved as the Beyer treble is, it's still a solar flare in comparison.
At least with the pleather, I never get a sense of a true spike. Very nice, refined, detailed, articulate treble, but I don't get a sense of that spike, and I really must insist that the Denons (maybe D5k does have sharper treble than 2k?) are by far brighter, almost in the Beyer sense. But yes, there's no question, once you've heard Beyer, nothing else short of Grado or HD800 will ever seem bright again HE-400 is simply not bright, in any way. To call HE-400 bright would be to call HD650 and K702 "Beyer's new lineup"
While they're relatively inefficient by headphone standards, they're efficient for ortho standards. Virtually anything will drive them. I won't deny that Lyr's my favorite, but my O2 drives them very well, in a different direction. Even little E11's will drive them, though not as wonderfully as a higher end amp.
LDMKII is nice, but as an OTL, it may not be the best match for low impedance headphones. It'll probably work, but I can't say how pleasing you'll find the output. Generally OTL amps do better with higher impedance cans and can't generate the somewhat higher current required by low impedance cans. But since HE-400 is more sensitive than other orhos, you still have a chance of it coming out ok!