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.
So I tried out both the HD800 and HD800S. The HD800S is clearly superior, in my opinion, to the HD800, if you're listening to both right out of the box. But I read that the HD800 can sound exactly like the HD800S if you properly EQ them.
So I got a trial version of Sonarworks True-Fi and tested the theory. Both headphones benefited from this software. In fact, the HD800 became superior to the HD800S (again, in my opinion). The main difference for me was that the HD800S could not reproduce the lowest notes in some of my favorite music, even with the bass cranked.
So then that got me to wondering: what could True-Fi do for the HD700, and their rather wildly erratic frequency response curve? So I bought another pair of HD700 to find out. Let me tell you, the difference was HUGE. You're exactly right: that big dip at certain frequencies is what caused my perceived loss of detail. With True-Fi restoring that detail, and taming the harshness of that bizarre peak around 6 kHz, the HD700 sound amazing! In fact, I like them better than the HD800 and HD800S! Yes, the soundstage is a little less wide, but I've surprised myself by preferring it. The 800 and 800S make everything sound like you're listening in a big echo chamber. I'm sure that's the reverb that was deliberately added in the recording studio. But the 800 and 800S make you SO aware of every detail, it gets to be almost too much. (Again, this is very much just my opinion.)
I'll be honest, I'm surprised that anyone can like the HD700 straight out of the box. It is such a unique frequency response. But, hey, we all have our preferences. I just know that the combination of HD700 and Sonarworks True-Fi provides "reference flat" frequency response in a very comfortable and affordable package. I highly recommend that everyone who likes the Sennheiser open-back sound give it a shot. Especially older guys like myself, because the Sonarworks includes the ability to compensate for age-related hearing loss. Just punch in your age and gender, and it makes the adjustment. You can then use a slider to dial in just how much of that compensation you want. It's beautiful! Music sounds like it did in my younger years again.
Thanks for your help! Your advice got me on the right track.