After much deliberation, I went out & picked up the c751's yesterday & have to say... out of the box, they were very good... with the excpetion of the tips
It took me a while to get a good seal, and the Medium "ideally" is the right size, but it takes alot of time to set it in...
So I had an extra set of T400 from my last EX300 & NC33s, and slapped them on... well they fit quite well and give better isolation thank stock and "I do belive" tames down some of the sibilence that is present in poor recordings off a portable device.
I've owned alot of headphones and not quite as many IEMs, but I did do a straight on comparison w/ the Sony NC33 & EX300 that I bought, and overall... well there is more width to the imaging, far more resoltion in all ranges, and mostly a deeper bass, yet retaining a somewhat analyticial sound.
It to me seems like it is a good monitoring can, and reminds me alot of my Sony MDR-V6/7506, where as the EX300 & NC33 are warmer like a set of KSC-35's/PX100s
"I" personally am a resolution nut, and not a basshead... I love grip and detail, but though the bass is deep and plentyful, I do truely enjoy these cans.
Other IEMs I've tried are the Skullcandy Titans (now my room mates

), Shure 210E, and some low end UE
Personally I hate deep seated IEMs, which is why I went Sony first...
Thing is... w/ the Comply T400, they are supurb, but it is a bit of pain having to roll them every time you insert them & they obviously don't have the best durability...
Should I pick up a set of Sony EX? I have a set of Hybrids that "I" should try, but it seems like the center core is way too small for the Denons :P
Has anyone modded the Sony hybrids to fit?