Momentum's frequency response measures almost identical to the HE-500 from 1000Khz downwards: putting aside the HE-500's brighter (neutral) nature, to my ears the closed back design of the Momentum naturally creates a much bigger punchier bass.
If there were more bass it would certainly lean even further away from neutral, which I think would ruin most acoustic recordings.
Welcome to the club - bassheads need not apply.
I would definitely throw the Momentum into the basshead-lite category. Lows are definitely emphasized a tad. Enough for me to not call it neutral, at least.
And all you've done is sell me even more on the HE-500. Thanks a bunch for that.