To summarize, the Momentum On ear & Over ear don't seem to target the same audience in my opinion and the on-ear is not a "cheaper version" of the original Momentum. The sound signatures are vastly apart; On-ear has a bassy (to put it mildly!) presentation, bass is slightly loose as well, pace is good. The original momentum is much more refined, good bass (refined and tight), warm with good treble and a relaxed presentation. Frankly, if one prefers a fast and punchy style of presentation, the Amperior is a much better headphone than the OE Momentum! I still don't see why Sennheiser went with the OE momentum when they could have just redesigned the Amperior.
Probably because there were a lot of positive reviews associated w/ the Momentum brand name, and it's new & exciting hahaha. Amperior is old & boring. People nowadays want the new thing! =P