I am A-B-ing them once again, and I must correct my comments before. The X3 sounds clearer, with a darker bass response. The iPod 5G sounded a little tinny, a little weak (the prime reason why I wanted a more powerful DAP to run the W4s). Both iPod and X3, however, sound fairly warm when paired with the W4Rs. I suspect I was unable to immediately discern the difference because they both sounded quite laidback.
Right now, I've tweaked the Treble and Bass settings on the X3 to attain the sound signature I was seeking. I understand why many members say that the X3 is synergistic with the W4; the warmth of the X3 and neutrality of the W4 do complement each other quite well.
Edit: I revisited my UM2 and realized what I was looking for; a SQ with the bass of the UM2 with the mid/treble of the W4. Oh what fun with Westone products.