I realize this thread is getting a little stale, but to the gentleman who just picked up a set of Amperiors and is wondering whether the T51i is worth the extra money, I would have to say (from a purely sonic standpoint), absolutely - YES! That being said, as I sit here awaiting the return of my beloved T51s from being repaired, if durability is a significant concern for you, then you may just want to stick with the Sennheiser.
I use my T51i's almost exclusively in the office and I take great care in packing them away safely every evening, being sure to loop the cord very loosely as I don't want there to be any tension that would cause a short. Because, as I'm sure you know, the cord is hardwired. Even so, after only a few months of light and careful use, I have had both earpiece covers come unglued (taped) and fall off. And most recently, I put them on only to find that the left channel was significantly diminished in volume. Granted, the nice folks at United Sound Services promptly sent me replacement earpieces and have now repaired and shipped (as of today) my headphones, but this does not bode well for future durability.
Meanwhile, my Amperiors are nearly bulletproof. I've had them for about 4 years now and recently replaced the earpads. They're MUCH more robust but even if you do end up having a problem, virtually every component is easily repaired/replaced.
But with regard to sound quality, the Amperiors are not nearly as balanced. They're great for listening to rock music at high decibels, but the Beyers are much more refined. And, as many have pointed out previously, they extract tremendous detail without having to "crank it up".
There is definitely something special about the design of the T51i though. They render such a sweet and musical presentation that you just want to keep listening. In fact, I like them so much that I picked up a pair of DT1350 (gold edition) to have as a backup. And while they utilize the same drivers and same earpads, and same earcups, they're just not the "same". They're very good, don't get me wrong. But they're not as comfortable and, more importantly, not as musical. There's a resemblance to be sure, but it's just not the same.
And lastly, with regard to isolation, I find the T51's to do a tremendous job. I think they do a better job at isolation (and controlling sound leakage) than the Amperiors. They're also better than my OVER ear Sennheiser Momentum 2.0 which I hardly even listen to since getting the Beyers. And somehow they achieve this isolation with the least clamping force and the smallest earpads. Go figure!
