All of this discussion about sound variations between K models, Q models, and even between old and new. . .I'll just say this (mainly pertaining to new/old debate). . .obviously there can be variations in sonic signatures, from one production batch to the next, there will always be manufacturing variations. This will obviously affect the sound coming from pair A to pair B headphones. The differences should be minute, obviously depending on whatever their quality control standards are, and while all headphones they ship out are more than likely within technical tolerances, if pair A happened to be at one end of the tolerance spectrum while pair B were at the opposite. . .well then the differences when comparing will be greater. I imagine this, along with other manufacturing variations (hopefully also within tolerances), can add up to create difference sonic signatures even among similar lined products. It's like a tolerance stack up, if you will.
One personal example I can share pertaining to my reasoning would be this:
My K701's seemed to have an imbalance between the R and L channels when I initially received the pair. Now the engineer in me had to rule out all possible factors and see if I could determine the root cause. After hours and messing around, and thinking I had exhausted all possible causes, I was just left sitting there staring at them resting on my computer desk. Then, almost chuckling, I thought, "well, I've not tried swapping ear pads between L and R." Sure enough, it was some sort of variation in the thickness/density of the mesh material in the middle of the pads. I contacted AKG and explained the situation, which they promptly, and quite generously I might add, sent me a replacement pair of ear pads. After receiving them I tested them, and ironically they ALSO created a variation in perceivable volume between L and R channels. Luckily mix-matching the two pairs of pads resulted in two sonically matched sets. Also, as a side note. . .it turned out to not simply be a volume difference, each set of pads actually "tweaked" the frequency response slightly (which is what made it sound like a volume discrepancy with a mismatched set of pads). One set of pads actually created slightly softer highs and slightly more low mids. It was, however, more than enough for myself and two other friends of mine to hear it immediately between the two sets of (now) matched pads. The two friends were oblivious to the fact that the sets of pads had any sonic variations prior to them testing, but both pointed it out.
Anyways. . .manufacturing variances are always there, it all depends on tolerances and matching. . .which inevitably creates variations among similar production models.