Thanks to DavidM's 50+ headphone guide I decided to un-mod my HD-600 today and give it another listen. Still don't like it for the same reasons as before. I just find it's sound makes me a bit sleepy.
Strangely enough it's mids sound as forward as my K601 (except for upper mids). I find the K601 mids to be flatter than those of the Q701. More balanced.
The HD-600 is supposed to have forward upper mids, but it sure doesn't sound that way to me Someone recently said the HD-600 had very forward mids and I don't hear it. I think they're quite neutral.
I put the foam mod back in place and it's back to it's old self. Just how I like it. I really do think it sounds like an HD-598 on steroids. Maybe it's good for those that prefer a slightly more energetic sounding HD-600. Not sure. Highly unlikely, but maybe it has some similarities to the HD-700? It's soundstage is still smaller than the HD-598, but no big deal. The mids with the foam install are slightly more up-front. This makes the sound a bit more engaging, but not so forward to be annoying.
I won't get into a cable debate (since the ODAC makes it hard to hear any differences), but I prefer the stock cable to the HD-650 one. I always got the impression that the HD-600 was bassier with the HD-650 cable. Maybe due to it's thickness or something. The lower mids feel more present and the mids seem a bit fuller.
Right now the modded HD-600 is slowly edging past the Q701. The HD-600 modded is still a bit more forgiving for harsh tracks than the Q701. Modded I actually think the HD-600 has more detail! Without mods, something about it's signature makes it seem less detailed than it really is. Maybe the mid/upper bass.
I think this mod sounds similar to the HD-600 with no grill. I have absolutely no clue why! Why would closing off an area of the back of the driver sound similar to removing the grill? Bizarre. Both give you a much clearer sound.