That's three professional studio/broadcasting cans with some serious staying power. Should be a fun comparison.
And it was.
All three are excellent, just with varying sound signatures.
7506 being the brightest with most vocal range emphasis, Beyerdynamic DT250-250 with the most laid-back sound signature, and most smooth/soft texture, and the V6 in the middle, very flat thru the entire sub-3k range, shaped similar to the ideal curve, but with bright treble.
7506's and V6's unmodded are actually slightly too bright and sharp, but with the DT250 velour pads on either Sony model, it softens and smooths things out, but doesnt make them quite as laid back as the DT250-250. Very good headphone, but not quite bright enough to win me over.
Turns out the soundstage increases gained by modding an MDR-V6 or 7506 with beyer DT250 pads, gets you pretty much to the soundstage of the 250-250 (which isnt great, compared to, say, an AKG open back). But the Beyerdynamic DT250-250 couldn't match the detail of the Sony models. So, I'm still rolling with V6's for general closed back needs, and 7506 for closed-back vocal tracking or vocal reference. The Beyer DT250-250 are just too laid back compared to both Sony's. DT250-250 is much more like a Sennheiser HD600, in sound signature (neutral, good mids, and laid back, easy to listen, than other Beyerdynamics I've liked (DT880, DT770, T90).