For starters, I was comparing the AD700's EQ'd bass to HE-400's bass un-eq'd when I made that statement. When EQ'd, I can get the AD700's perceived bass volume at something nearer to HE-400's but it sounds like a bloated mess with no punch or texture in comparison to HE-400's. It also still doesn't go as low even with +6db at 32hz.
We're comparing apples to oranges here. HE-400 is open backed and has bass that's relatively neutral. (Albeit very powerful for being close to neutral) Pro900 is closed back and is one of the most colored bass-oriented headphones on the market.
As you can see here, the Pro 900-- by design-- has +15db of bass boost that's centered around 50hz. What the FR graph doesn't show you, is that it also has a rather high THD in its bass, possibly giving it the effect of another additional 3 or so db in the bass region.
So, to make it a more fair comparison, you should EQ the bass on the HE-400 up 15-18db to see if it's stronger than your Pro900 (you'd need an amp extremely powerful to allow for enough headroom if you were to do that) Likewise, you could reduce the Pro900's bass frequencies by 15db with an EQ as well. You'd also need an EQ that supports that kind of curve too. For me, iTunes EQ sounds like **** if you reduce everything by more than around 7-8db to boost your bass by that same amount. It gets dark out of nowhere. That said, with only 6db of bass boost using iTune's EQ for me, HE-400's bass is way more than I'd ever need.
Edited by TMRaven - 1/15/13 at 10:17pm