The AKGs aren't going to enjoy being un-amped. In fact, I bought the Shures and believe, at least the 440s, can run off an ipod *maybe*, but no way on the AKGs. They are easier on any amp you use, or pre. They are perfect if you are plugging them into a headphone out on a nice home audio pre/amp/receiver, and they work alright off my E-MU 0202, they work better on the Behringer headamp. So do the Shures, as they really reach up into the HF so I am happy to dial down the trebel on the Behringer about two notches or so. I do not need to do this on the AKG240s.
Presumably, the Denons will do better here as well, and the Sony XB700s don't really need it, but I jave 4 amp sections, so... but they sound nice without any EQ. I have not really had an urge to plug in my HD555s yet, as the AKGs sound so much better than the HD555s when they some muscle behind them. Smooth as butter, is what I think I blurted out last night when I got everything hooked up. Prior, I was using a different DAC, but even with the better DAC, same thing applies. The AKGs really want a headamp, and if you just have the portable player, the Sony XBs probably sound better than anything other than PortaPros. The HD555s do alright on a portable player, as I remember from about a year ago when hurricaine Ike hit. I had time to jack around with various headphones with my walkman. (I was making mixtapes still for a while because it was the only way to capture the sound of my full range hifi in a portable).
Hope that helps. a lot of the design of the Shures seem to be toward working in a studio enrironment, or just on the road in recording/monitoring. They weigh a lot, are built well, and they handle a ton of power. The AKGs are really good hifi headphones that probably sound great in any studio, or any good headamp. Right now I am listening to them and really enjoying how far the sound extends beyond my head, as well as the clear HF. The Shures have a more neutral sound. I'd say Shures are great all rounders as they probably only need an ok headphone pre, or portable amp (booster?) to sound their best.