While testing the HM-601 with the Ultrasone Pro 900, I got positively surprised.
My Pro 900 are not modded, so usually they sounded rather bright and had sharper highs. These sharp highs are the reason for the Ultrasones being a dustcatcher in my room. However the HM-601 let Ultrasones sound little less bright and the highs are not that sharp.
So genrally I would describe the Pro 900 - HM-601 combination:
Highs: Sharp like a Razorblade, but more gentle like that Gillette shaving stuff, I mean you can still get cut, but rather rarely.
Mids: Are present and very nice. (Still not like the ESW9, but ok)
Lows: Good and do not overwhelm the mids.
Big soundstage and very detailed.
May be it is because I like new stuff more and the fact that at last I can use my Ultrasones... However this is my new favorite combination.
#1: HM-601 & PRO 900
#2: Cube C30 & ES10
#3. everything else
EDIT: While I was writing this I listened to Capsule - Jelly (Remix Version)