Just did some extended listening this morning using my ipod touch 4g, 5g, classic, note II, and my laptop. Without the amp, the laptop sounds the worst by far. 4g and 5g touch sound pretty similar and honestly so does the classic. The note II sounds better buts that's probably down to using flac files and a better media player.
I can't really hear all that much of a sound difference between running amped vs unamped apart from the loudness (you need to crank the volume all the way up to get decent sound out of most of my devices). Bass doesn't seem any more prominent when using my O2 compared to just straight out of my phone (which is to be expected I suppose since the O2 is meant to be transparent). The only major exception is when I run the 650's straight out of my laptop 3.5mm jack, it just sounds tinny (very light on bass) which is probably due to the crappy integrated audio.
Anyways these are my results with my untrained ears, someone more experienced might be able to help you better.