To add to this discussion on SQ and scaling, my experience from comparing HD650 sound characteristics (new silver drivers) from two different amps. . .
On the Bottlehead Crack OTL tube amp, the 650 is very warm, subdued, not very detailed. Not unpleasant at all, but there's less excitement, seemingly slower response, and less treble. Some might say the sound is veiled, but I wouldn't necessarily describe it that way.
But when driven by a Hifiman EF5 hybrid amp (modded to use AD797 opamp vs stock OP275), the 650 sound characteristics change quite a bit. Still warmish, but I find them to have much more energy, detail, and faster response and restored treble. Couldn't see anyone describing them as veiled with this setup.
I also have DT880/600's, and if I drive the 880's with the Crack and compare them to the 650's with the EF5, there's not a ton of difference in treble, detail, response. The sound characteristics are still different, but both setups are very pleasant to listen to and I don't feel like I'm lacking in anything. The 880's have a bit more detail, but it's certainly not night and day. Of course, if I plug the 880's into the EF5, I get the most detail of all - I don't find it fatiguing either, but some may.
The point being that if I only heard the 650's with the Crack, I might not be fully satisfied with the sound and dismiss them as insufficiently exciting, etc, but I am very happy with the EF5 driving them. There's less of a difference in 880 SQ between amps versus the 650, which shows a more significant change.
So after my experience, I do buy that the 650 can sound significantly different depending upon what's driving them. It may never reach the level of the most detailed/analytical cans, but I don't think that's what it's shooting for either.