I own and rather enjoy music w/ my MDR-V6's ...however the fixed long coiled cable makes them something of a disaster for mobile use.
Personally I think of the choices you have mentioned the ATH-M40X's are the best as they have the detachable cable (comes with 3 cables I think ...or at least that what came with my M50X's (long coiled, long straight and for mobile use short straight).
The M50X's are honestly more robustly built, but the M40X's are not bad in that regard ..probably on par with the V6's .. The M50's probably have more bass than you are looking for ..but the M40X's have less and many seem to like their sound more.
I think the removable cable really makes the M40X's what you should pick.... I sometimes take my V6's out and about with me and the long coiled cable is just a drag.... and a tripping hazard sometimes
. I think I will do a removable cable mod on my V6's soon because the coilded cable drives me nutz.