One thing I should specify is that out of all my friends in that group I am the one who probably listens to the most hip hop or electronic music.
I think we were all taken aback by the bass response on the On-Ear, considering that it is stronger than the Momentum (which is pretty neutral in terms of bass) and the Amperior (which is actually has a slightly elevated bass response). For the music that they listen to, it wasn't a great match. Even for electronic music I find it a bit overbearing at times. The scooped mids also bug me a little.
I can tell you that I have been listening to them since then and my opinion hasn't really changed. Burn in or not, no one has ever claimed that burn in dramatically changes the basic signature of a headphone. If it did, they would probably burn it in in the factory since most people buy or return things based on first impression.
My opinion is heading towards the idea that the On-Ear is a well built headphone with a articulate and lively sound but you have to really, really like your bass. In many ways it seems like the antithesis of what the over-ear Mometum's signature stands for.