I am currently using Sennheiser IE7. While it has great bass, good sound stage and relatively good sound quality, its treble is way less than impressive. And also it lives up to the Sennheiser signature-- the "dark" signature that may cover some important details. Hence, a three-way triple armature driver is the way to go.
I actually auditioned 3 IEMs before deciding.
1) Westone 3-- It has 'thick' mid-range and stronger bass, but some details are missing. The sound is not neutral (slightly colored but not like Sennheiser), which may not be a bad thing. I thought I accidentally enabled the equalizer, making it sound "colored".
2) Ultimate Ears Triple Fi 10 Pro-- Very poor fit and isolation. The bass is tight, but not deep (to the point of near non-existent-- could also be due to fit issue). Mid-range is rich, but its the treble that wins. Sound-stage and instrument mirroring is incredible, easily the best I have ever listened. I remembered turning my head to both my left and right as I thought I heard spectators clapping beside me while listening to Yanni Live at the Acropolis. I will remember this great set of IEM as having the widest sound-stage I have ever heard!
3) Westone UM3X-- Neutral is the word. Not as "colored" like Westone 3, deeper tighter bass but not over-powering like Westone 3. Sound stage is slightly weaker as compared to the other 2 (weak but still pwns IE7), but it sounds very very precise. The treble just slightly loses to UE 10 Pro, but still considered to be outstanding. I can certainly hear lot of details. This will be great for classical music.
All 3 are tested without any prior knowledge on them, and they just fitted these IEMs into my ears so I had no idea which is which. The staff at CE smiled and nodded their heads when I shared my mini-review with them.
Amazingly, UM3X is the cheapest among them. SGD$529 compared to Westone 3 ($559) and UE 10 Pro ($629).
So end of the day, I switch to UM3X-- and never look back!