ibasso T1, fiio e17, I hear so many people raving about these http://aloaudio.com/alo-audio-the-continental-mobile-vacuum-tube-amplifier.html . the continental.
or just get cmoy 3 channel from fred fred 2004 on ebay, they are like $150 and they sound like >$200+ amps
The Arrow 4G or FiiO E11 are highly recommended with the W4s. Very clean, effortless sound with low distortion and a Iack of coloration that often happens with sensitive BAs and improper impedance matching. I also found they sounded great out of the iPhone 4. The DX100s were good, but a bit overboard for these IMO.