I let the Prime run in with sweep tones during the day yesterday, and also swapped out the stock cable for the Moon Audio Blue Dragon.
The Prime sounds overall quite neutral to my ears...I don't really hear anything would want to change. I don't hear a lot of sub-bass but most recordings don't have much to begin with, and I don't really expect a lot of it from that driver. I don't really care either, since everything else is as it should be. The sub-bass will probably show up on it's own when required. I'm just glad it's not of the forced variety.
The differences between the Prime and the Alpha (to my ears)? The Prime has a much more 3D-like image, it has better top-to-bottom height in that image...things are stacked with space around them. The AD seems more center-filled, and is also a bit boxy sounding after the Prime.
But again, I am very early in, and I think the Prime is one of those headphones that one has to "grow into" in order to fully appreciate what is does and has to offer. You are not going to hear your "$400 upgrade" right off the bat...I think it will show itself in long-term happiness