Just got the 700's and wow I am impressed. Definitely a different signature than the 650's. I need to get these puppies broken in a little. The Pan Am does a super job with these phones.
On the low gain setting i am running about 10 o'clock on the amp. I can't believe I am hearing stuff I wasn't hearing on my 650's. That may be a bad thing on some lesser recordings but I think I have found my end game set of phones (for now lol).
Congrats on your purchase! They are definitely more analytical than the HD 600 (and 650) which means you're going to spot more errors and artifacts on poorly mastered or encoded material. Still, it's nice raising the bar on part of your line because it forces you to improve all factors. After my purchase of the HD 700, I did a massive overhaul on my content, replacing all 320 kbps material with lossless material. When a headphone makes you want to be a better audiophile, that's a good headphone.