I have the same setup as you (JBL LSR305 + PC) and just bought a used EMOTIVA XDA-2 at a local craigslist.
I can tell you there was a great improvement in SQ (better imaging, sound stage, clearer notes, etc)
I hear things that i wasn't hearing from w/o the DAC...I feel like I'm finally taking full advantage
of loseless files I've been collecting.
As far as aesthetic goes, I wish it was little bit more subtle and simple, but I can still live with it.
It's a matter of personal taste, so you will have to check it out for yourself.
If I were to get another DAC/preamp/headphone amp for your budget, I would look into
NuForce uDac3 or Audioengine D1 and spend the rest on the headphones.
