"The 712 is a little more laid back,"
I prefer this because you can turn them up louder.
Agreed, the concept of "better" or "worse" sound is extremely subjective. IMO, the HD600 does no wrong, but at the same time, they may be very boring to some people as a result, and not what they're looking for.
EDIT: just saw your reply. Sound quality itself is also just as difficult to define...what sounds good to one person isn't the same to the next. There are very few objective factors in this.
K712 sounds like something you would like then. I own a Q701, and have heard the 712 has even more detail in the treble and also not quite as forward mids, but it's bound to be a much more exciting can than the HD600s, which don't have nearly that kind of head stage or exciting FR. It will certainly be a new and higher quality overall sound than the M50s you're coming from in that case.
To a bass head they might.
This is getting stupid.
let me ask you something....
When you play your stereo at home and in the car...do you turn up the treble and/or bass or do listen to it flat?
I'm also thinking the Senn HD600 might be your best bet unless you want powerful bass thumping your ears (bass head)
The HD600 is used by a lot of musicians to hear exactly what was recorded (reference headphones)
People who think the HD600 lacks bass prefer the HD650.