How are they not the same given the same drivers/housing?
The housings aren't identical and they've changed some other things - you can actually see this demonstrated with measurements between the K701, K702, and Q701. The 702 "fixes" a treble ridge in the 701, and the Q changes the tonal balance slightly. From seeing a variety of threads, the Q probably has the most popular appeal, and the 701 is probably the most analytical sounding.
http://en.goldenears.net/index.php?mid=GR_Headphones&search_target=title&search_keyword=701&document_srl=4138
http://en.goldenears.net/index.php?mid=GR_Headphones&search_target=title&search_keyword=702&document_srl=8281
http://www.innerfidelity.com/images/AKGQuincyJonesQ701.pdf
http://www.innerfidelity.com/images/AKGK701.pdf
Thanks guys! I have given thought into the AKG 702's and the Sennheiser 598s, both of which I have heard great things about. Which of those would you say has the bigger soundstage? I also thought about the Sennheiser HD 650 but I wanted change up from a bass heavy sound and I don't think that my Fiio E-9 can drive it. The DT 880 on the other hand is something I had not considered and I will delve into some user reviews on it.
The Sennheisers are not "bass heavy" really - they're closer to neutral, while the 70x is more "thin" by contrast. Compared to the Denon, the 70x is a very big departure. The Sennheiser are more complementary imho. E9 will drive them fine (it will drive 70x fine too, and the DT880, etc; none of them need *that much* power). Overall, I'd do the Sennheiser 580/600 (the 650 is supposed to be more dark; might be fun). I'd also look at the Ultrasone HFI-2400 (even darker, more bass too - also cheaper).
Grado is a fun suggestion as well - will be a VERY big contrast, and certainly worth looking at. The SR-225 is still the entry-point imo (I don't like the 325). Comfort will be a very dramatic departure too (the Denon are one of the most comfortable headphones out there; the Grado are...not).
Finally, I want to suggest a Sony open-back headphone, but the models I've heard/like are all discontinued; you might want to follow the MA900 thread as they become more available and adopted - it might be exactly what you want in terms of "airy and mid/treble forward." (The MDR-F1 and MDR-SA5000 both would do that for you, but they're both discontinued tmk).
I'm not a Beyer fan, and don't care for them, so ask someone else about them (if you don't have anything nice to say...).