The previous poster is right in context, the D1100 is more balanced in the mids, but a less punchy and clear bass in my view than the D2000
That with the E1... or un amped.
Amp both with the Voyager and the transformation affects both.
I can't quote much else - because thats all I've used apart from an expensive homebuilt 120W MOSFET amp that's very flat from 2Hz to 40kHz. Even at the lowest setting, on that it was clear that the 2000's were much more capable than the 1100s. Not very easy to hook up - and certainly not portable with the weight of the two PSUs needing their own fork lift!
With the Voyager after nearly 6 months of use, the 2000 keeps the deep, seemingly bottomless bass, has a nice responsive mid bass and now has a real 'punch' in the mids. The highs are not as strident I find as they have been described.. but perhaps that's my hearing (lots of rifle shooting has affected my high range perception a bit - but not that much!) A very balanced and rounded sound, with a great 'forward' sound stage. Maybe not at the Senn 650 level, but then not at that price either!
The D1100 gets the similar benefit, but the bass is not as bottomless, the mid highs not as clear and the mids a little bit less. Still a good phone though.
I have to say I need a more closed phone, to stop disturbing the other half - I'm an insomniac so listen in bed after she's sacked out. These still leak sound but are much quieter than the open phones. I reckon at this price point though, that these D2000s are as good as many open phones costing almost twice the price.
I'd really consider the D2000's try them with a cheaper FiiO amp, and if you can later upgrade to a Voyager - do - they really work well together.
I'd agree with previous posters though - if you really want to hear what the 2000s are capable of - then you should use a decent Amp.