Thanks applebook! Unfortunately you're right though, there is no way of telling.
I could well have been sold HD650's with HD600 drivers inside. If it makes any difference, I definitely have the 'silver screen' version of the HD650 that is said to be brighter than the 'black-screen' older version of the HD650. By 'brighter' I presume that means the 'silver screen' version have less bass and more treble quantity than the 'black screen' version.
Even in this thread there is disagreement, with applebook saying no way would the DT880 ever sound as bassy as the HD650, and others saying that the bass is the same, but the reduced treble makes the bass look more in the HD650.
It would be a great help to me if someone did a quick A/B (if they happen to own both HD650 and DT880) comparison using a named track and on the same amp, and they could tell me what they notice in the bass (and, indeed other parts of the sonic presentation as well if possible) that is different between the DT880 and HD650...?