Cross-post from the FiiO E17 thread, as the D3 is responsible for more the positive outcome than the amp anyway...
For what it's worth...
I'm never going back to [Source -> analog headphone or line-out -> E17 line-in], since getting my Cowon D3 and its proprietary A/V cable that I bought to drive the E17. It enables me to stream digital from most any type of files [preferably FLAC] to the E17 via a S/PDIF coax. And better still I was so very happy to discover the digital EQ and BBE+ effects performed in the Wolfrom DAC are all in effect over the coax out [i.e. the pre 'AC' part of the monolithic Wolfrom DAC] ; based on the block diagram of the DAC chip I thought the S/PDIF out was before the effects section but that proved not the case. So I can use the semi-parametric EQ of the D3 to do fine tuning that may be needed for the phones I'm using @ the time, but leave the overall treble/bass tonality 'finishing' of the sound to the E17.
I use the D3/E17 combo w/my Focal Spirit One headphones the most, which I think is a very overlooked headphone but IMO is the most engaging & do-all / capable headphone that is brought alive with small tweeks of the D3's digital semi-parametric EQ, some JetAudio enhancements, and the E17's power. Simply amazing "crisp -to- head shaking" sound from the whole package and quality music tracks like FLACS of the whole re-mastered Steely Dan collection, or of say a 2-channel PCM rip of the Cream @ the Royal Albert Hall DVD, or board recordings of concerts mastered and sold by the artists. I don't think I've ever had the "I'm @ the concert' experience from headphones like i have now; especially given at least one set of FLACs I have is from when i was @ the concert and I can compare what I hear now to what I heard there when live. And the E17 plays a vital role in achieving that. Anyone in the NYC area is welcome to hear what has me so over the moon @ the upcoming meet in Bablyon.