I think the Momentums sound much better with an amp.
Straight out of my iPad and my iPhone5 the bass is just a little bloated. When I hook them up to the Yulong D100 or my Fiio E7 the bass is tighter and...err...just better.
The DT1350s sound just as good with or without an amp.
I'm happy with the purchase. I can understand why this is one of Tyll's favorite headphones.
I can definitely see that. I have my iPhone 4s volume cranked to the max so the sound can be where I like it, if I mess with the equalizer at all it's quieter than I want it. I've just been avoiding sinking money into a portable amp because there are other things I need my money for right now.