The Westone 4 is warm, but unfortunately so much so that it is bulky sounding....I would say the low mids especially in the 250hz to 400hz region are present in an alarming ratio relative to 1khz and this facet causes the device to sound muddy; that obfuscates the imaging , stereo field and kick drum/ bass guitar timbre. In comparison, the ue900 extends far lower into the sub bass region, is less muddy and opens up the stereo field to a significant degree....the lower mids are slightly enhanced which gives the warmth and fullness that each vocalist wants to hear, yet those frequencies are supplied in a fair ratio with other mids and highs so as to not mask their presence.....the ue900 high frequencies extend further than the Westone 4 banger and add a glistening and presence that really brings the space around each instrument to light and allows the appropriate reverb tails to be heard....not a reference, but a bit more enhanced....not quite like a loudness function though....the highs on the ue900, to me, are spectacular and better than any iem I have ever heard.
In comparison to triple fi 10, the ue900 is more generous n the low mids which give rise to an extra warmth, but also it has further high frequency and low frequency extension....clearly the highs from 5k up are more present and add air and instrument clarity and depth. This is not a low frequency mondo boosted device, but the extension seems almost flat down to below anything I can hear which gives a visceral and physical component to the sound and experience.
My 0.02, subjective as it may be.
I agree, too much lower mids on the 250-400hz in the W4, too thick in relation the recording, the midhigh dips are a bit of a shame as well, though to a lesser extent. You know the TF10 is one of the rare BAs to have sufficient rumble, but I suspect most can't insert them well enough, far too bulky (I could only achieve it by holding on to the housing LOL). Good to hear about the extension on the subbass and higher treble on these 900s, it's quite a feat as the TF10 was no slacker. The W4 doesn't have outstanding extension on both ends considering the amount of drivers used, still quite good though don't get me wrong but there's more potential.
Edited by Inks - 9/6/12 at 10:48pm