i bought both the UM40Pro and the W40 recently. kept the W40 and returned the PROs. both sounded good but i thought the W40 was more neutral. the UM40 are tuned for musicians and the big difference is that any percussive beat is front and center - i didnt find it boomy, more like the drum kit was at the front of the stage and a little too loud.
the W40 are a bit more balanced. a great IEM that provides more detail than my ety er4p. they are a tad flat right out of the ipod but still sound damn good. anyway, an expensive but good headphone, lightweight and low microphonic cord. also, first silicon tips that i've liked.
i will be using the w40 as my daily do everything headphone and will not be using my trusty ety anymore. tho i still believe that for the price the ety is the best deal going for iems.