$800 is a tough spot for a complete setup to be a big improvement over cheaper solutions.
If you've got a mac, then a duet + senn hd580/hd600 or grado sr225, if you don't have a mac, then replace the duet with a pico.
Or you can maximize amplification instead of source quality, and go with a headroom desktop, m3, ppx, millett max, gilmore lite, etc and a decent pc usb soundcard.