UM3X is better in all aspects except vocals, which are a bit more upfront/warmer on the SM3 due to mid-centric sound sig (but still amazing on the UM3X). Instrument separation is better, more clarity, more details and better treble (quantity and its more sparkly). Treble on the SM3 is indeed recessed. If you A/B both, you almost can't hear the SM3's treble. Bass quantity and quality is same thing. UM3X is brighter, SM3 is dark. This is after an extensive A/B. No doubts, UM3X wins hands down. Going to buy a silver dragon cable for it and it will be heaven, with even more details, better soundstage and deeper bass.
Edited by miow - 12/9/11 at 3:06pm