I haven't really kept up with any Ultrasone threads in a while. The Sig pros seem to be fairly well received (more than the editions at any rate). I've heard the HFi 450 (as a rebranded Yamaha), and Pro 550/650/750. S-Logic just feels a bit weird to me overall, though I enjoyed the visceral rumble from the Pro750. I'm curious about the Pro900, but the 750 was verging on too much for me so I'm not sure if I would like the 900.
The M100 just didn't suit my ears. It sounded all over the place. Most of the time when I play a frequency sweep I can clearly tell where the peaks and valleys are, but then in listening to music it's much harder to tell aside from general bass/treble heaviness. The M100 on the other hand made me do an immediate "what the?". Just the way the response played out was really odd to me.
Right now my bassy needs are served by my modded T50rp and the HE-6. Both have J$ pads on them which gives extra rumble in the bass.