What does "quality" mean in this discussion?
- The luxurious feel and image of the materials used?
- The fit & finish?
- The long-term quality and reliability?
The overall best build headphone/amp ever made is the Sennheiser HE90/HEV90 - no surprise given their price...
The HD800 is underrated because of it's "spacy" look and silver painted plastic. But given the robust design, precise fit & finish of nearly every detail and the use of expensive and rigid materials mostly in invisible areas (normally it's vice versa...).
The T1-prototype I held in my hands on IFA was superbly build, but they seem to have trouble with production units - that's why they're postponed, let's see.
Many people may see it differently, but just put it this way: we won't see any convincing Chinese copies of neither HD800 or T1 in the future - their production is too complex (just try to find a supplier for the mesh).
There are other luxurious exotics, my (unlike other Stax) Omega 2 is also well made - but not very well thought out...
The DT48 is a typical professional headphone, very rigid - but neither beautiful or luxurious...
But many so called "luxurious" headphones are designed to look luxurious and I'm not too convinced about their actual build-quality, Denon, Grado, HE-5, Jade... is typical for that.