My original comment was that the Ed9's bass was more boomy in comparison to the 750's. And I stand by that. (and I think you might agree???) The sum-total bass level of the Ed9 is obviously mountains greater than the 750's...In order to extend an olive branch, I will grant that I might be confusing "boomy bass" with "strong bass emphasis", if you know what I mean. Too much emphasis? My gut says yes, but I could be wrong.
This would be a technical response to why some may hear a boominess with the UE9. Use an amp without the built in equalization and you may find a different result. This is not to say that the amp is not flat under standard test conditions but with a dynamic load where the load has large impedance swings that amp if flat then will not be with a load that has a fairly flat impedance curve.
This speaks to synergy. In my gallery if you can get to it are several curves showing various headphone impedance curves.