To my ears, they are not even close, particularly the harsh, grating highs on the Pro 900 reminiscent of nails on a chalkboard compared to silky smooth, if less extended, highs with no sibilance on the M-100. Bass is more prominent on the Pro 900 but more controlled on the M-100. It's not that I don't like Ultrasone- I am a fan of the Edition 8 and Sig Pro, but the Pro 900 is actually painful to my ears with such a dramatic v-curve.
I second... just about every word of this. (I've never owned the Signature Pro's, however)
I assessed the treble range of the PRO 900 to exhibit a highly metallic sheen whilst the bass was presented as an indistinct cloud that formed OVER the top of the presentation with bass-heavy content. The M-100's signature isn't as dramatically v-shaped, it doesn't exhibit high-end harshness, and the bass stays in the basement (for the most part) and never becomes offensive. The PRO 900 earpads are comfy, however.