The SM3s, in my opinion, are like a louder, more playful version of the SE530s. The SM3s are definitely mid centric, but their mids (compared to the Shures) sound a little bloated and a little less realistic than the Shures (especially for vocals). However, the SM3s simply mop the floor against the SE530s in terms of soundstage. In my opinion, the SM3 bass is large but kind of uncontrolled. I understand why it's rated the #1 IEM on Headfi, but for me, the sound signature never really clicked. Everything you hear is great and you simply cannot lay any serious criticisms against the SM3s sound quality, but something about it seemed "too playful."
I suppose for those who like fun IEMs, the SM3s are probably the best choice. It bests the SE530s in terms of instrument separation, sound stage and highs. However, I find the SE530s have a more mature and solid bass signature, and its mids are fluid and realistic, while the SM3s (actually, across its whole spectrum) seem a little exaggerated. The sound is very very good, but the sound signature is definitely not for everyone. In brief, the SE530s sound more mature, in my opinion, while the SM3s are more for play.
Sonically, I find the SM3s are probably more articulate than the SE530s, but I reckon it's not a failure on Shure's part. I think it's because Shure specifically focuses on the midrange and wants their IEMs to be stage monitors for vocalists, thus rendering vocals as realistically as possible above all else (they are, after all, one of the leading stage mic companies). However, in consequence, the SE530s kind of neglect the air and instrumental separation some people love in other IEMs. As I said in my SE530 vs UM3X post, the SE530s give one solid sound, while the UM3X gives a more separated sound.
Speaking of that, when I first listened to them, the first thing I thought was the the SM3s were in the middle between the SE530s and the UM3Xs. The SM3s stole the articulation from the UM3Xs, but aren't as articulate or analytical, and also stole the midforward lean from the SE530s, but renders it very colorfully and exaggeratedly. So, you could say it excels at being the middle ground between the two. If you assume the aforementioned, I think you will be able to appreciate the SM3s capabilities, but not thoroughly enjoy them like you would your other IEMs if you really love the UM3X or Shure SE530 signature.
SM3: Playful, colorful, mid centric, huge soundstage, unrefined/uncontrolled bass, exaggerated, good articulation/separation, very good sound overall.
SE530s: Mature, very mid centric (otherwise very neutral), extremely realistic vocals, reserved and neutral "proper" bass , great lows, very fluid and natural, okay articulation/separation, mediocre highs, above average soundstage,
UM3X: Highly analytical, cold, very punchy bass, phenomenal instrumental separation and articulation, good soundstage, very accurate and sparkly highs.
Edited by Pepper - 11/14/11 at 5:01pm