I thought i would just provide some feedback with the C5D and the HD598. I am coming from a xonar stx and HD598, and the cowon j3 and westone 4. It is nothing fancy, but i have to say i was considering buying the HE-400 as an upgrade - however after hearing what the C5D sounds like, it is really a noticeable improvement for the HD598! I am no real audiophile, so I hope i get the terms right in terms of describing the sound. I tried this with classical (mostly), rock, pop.
IMO It noticeably opens up the HD598's. It sounds very airy with this and it seems a bit more accurate imaging wise, perhaps because of a bit better detail. I don't really think these 'add' any real brightness, it seems to be quite neutral. The bass boost (medium position) seems to give just the right amount for these headphones. It can be put on the max and the bass does not take over the music, it adds that extra punch for songs that would warrant having it.
I am hard pressed to find any major difference in the DAC performance (STX -> C5D (amp) -> 598) or (C5D (dac/amp) -> 598). I think the amp makes the biggest difference by far.
The real advantage IMO is being able to use this with my laptop as it was having trouble driving the HD598's which should be fairly easy to drive. In any case, this brings the same sound when travelling in a very portable format.
The westone 4 i didn't feel warranted an amp, but from the laptop again, its far better using the C5D. I wont comment on the SQ difference as i didn't hear it long enough. Nevertheless I am very pleased with it!