I found some Beyerdynamic Custom i Pro headphones I got a few years ago. They sound pretty awesome through Mojo. Much better than the HD700. So I'm definitely sending the HD700's back.
Any other headphones I should try out in that price range, with better synergy with Mojo? (Sorry: Maybe I should ask that question in another thread.)
Focal Clear, HD660S.
The issue you have with the 700 more likely has to do with its frequency curve. It has a pretty significant dip, one that instead of you perceiving as 'openess' more will sound like lacking clarity.
The effects of amping headphones is vastly overrated in my opinion, and a higher impedance amp such as the Sennheiser isn't really going to do your HD700 any favours. Even the HD800 should sound good out of a modern iMac headphone out. I know shocking! But I'm doing that right now and it sounds great - if not excellent. Some headphones really need some serious power to drive them (Hifiman HE6, electrostats) but the 700 certainly is not one of them. I'm sure others might disagree, but I would pose that any additional bloom / warmth tube amps might add to the 700 is just a deviation from their natural sound.
The other alternative is to just enjoy your Custom One Pro and 598! Save yourself some money as those good sets. I would check out an HD800 or HD800S sometime down the road. See if it is worth it.