Not to be a pain, but I don't see the FiiO e7 or the hd598's in your sig so you are making some pretty bold claims there with apparently no real world knowledge of any of the products being discussed. This is the a major problem with head-fi.
The hd598's are pretty much a tweaked hd595 which is not known for needing an amp to sound great, so I don't really trust anything you just said.
Yes it is a pain because what you could get with the HD595 connected straight to the DAP was mere 40% at best of their abilities when properly amped. Getting bass from the HD595 or HD598 is a simple trick. Put resistors in series with each channel. 75 ohms is a sweet spot IMHO but use amp first. With a right amp, there is no need to play with resistors unless you need huge amounts of boomy lows.
Edited by majkel - 12/25/10 at 3:19pm