I'll try to keep this as concise as I can, as I don't want to bore anyone. Anyway, starting at the beginning...it was a cold and stormy night. No, wait, that's a different story...
It actually began with a Sony ZX2. Lets just say that the output quality of it highlighted the deficiencies of the IEMs I was using. So I decided to try out the Sennheiser IE800s. They were a massive departure from what I was used to. Yes, I did find the highs somewhat harsh and sibilant. But the details, instrument separation and clarity was absolutely astounding—in my estimation, WELL worth a little harshness.
Fast forward about a year and my desire to get some full-sized cans that were of similar sonic quality to the IE800s had overwhelmed my judgment. After much searching, comparing specs, reading reviews, etc. I decided to get the Oppo PM3.
My immediate response upon hearing them for the first time? "Oh. They...they must need burn in before they sound good". Not a terribly auspicious start. However, I was more-or-less right. After 100 hours, I gave them another listen. In fact, I listened to them exclusively for about two weeks. And I did find them to be solidly good. A bit smooth and laid back compared to what I was used to. But certainly listenable.
Until I went back to the IE800s. The clarity and instrument separation were just astounding me all over again. There simply was no way I could switch between the IE800 and PM3. The difference in sonic quality was just too jarring for me. So, off to eBay with the Oppos. And in their place? The HD700.
Honestly, I was fully prepared to jump in the deep end of the pool with the HD800s. But these headphones are destined to be used with a portable source most of the time. And I just don't think any portable is really going to do justice to the HD800s. Given the lower impedance of the HD700s, as well as a multitude of reviewers who mentioned how they have rather harsh highs (a sign that perhaps they held a similar sound signature to the IE800s), I got a pair.
My immediate response upon hearing them for the first time? "Ho! Lee! Sheet!"
I find their signature VERY similar to the IE800. The two major differences that I've found so far are the sound stage and the highs. The sound stage is noticeably wider and deeper than the IE800s. There are some tracks where various instruments that used to just sit "in my head", now have spatial depth. I can "locate" their position either a little in front or behind me. And as for the highs, they are certainly sharp. But not as sibilant as the IE800s. Not sure if that's because of a difference in the drivers themselves or their position outside of the ear canal (or a combination).
And surprisingly, the ZX2—which has notoriously low output power—is actually capable of driving the HD700s. I've found that most of my tracks require 80-85% of max volume. 90-95% tends to make said songs too loud for me to comfortably listen to. But even so, I haven't detected any distortion at those levels. That being said, I don't doubt that some people with music recorded at a low levels and/or have some hearing loss might run out of steam on this particular combo before they reach their desired levels.
But, long story short, HD700 is awesome! If you love the HD700s you'd probably love the IE800s (and vice versa).