Just a question...is there any theoretical advantage or disadvantage of using such a small USB powered DAC over a full sized desktop DAC for around the same price...lets say 200-300 dollars entry level offerings? I'm talking about using it strictly as a DAC that would feed an amplifier.
Performance, performance, performance... :-) Also, no dedicated line out, no optical/coax input (if you need one, of course), and you are paying for a headphone amp that you are going to use as a line out (while could be OK, it is not exactly right).
DF is a very good little device but I would not (and do not) use it as part of a desktop only system (I use Arcam rDAC, and I would not try to compare).
To stay within your budget, I would look at something like this brand new model of Musical Fidelity V90-DAC (if you are OK with 24/96 and do not require 24/192; $299 at Audio Advisor). I owned their older DACs in the past and the performance was very good (although did not listen to V90-DAC yet).
Of course, there are a lot of choices so just try as many as you can, in your own listening environment, until you find something you like.
If you want to use as both though (portable/desktop) and like the sound, then why not?