what are your current headphones just because of tube vs SS sound. do you really want it to be a single unit or can it be separates. i personally would recommend separates for ease of upgrade in the future. if you are willing to buy used the headroom portable micro with DAC could probably be found within your range. also i would check out Meier audio for some amp/DAC combos.
a quick list of possibilities.
-headroom portable micro
-heed audio can-amp
-used nu-force icon hdp
-some Meier combos if you can find them for the right price
-maverick tube magic DAC/amp
some of them might be a little (of a fair amount) over your price point but you could probably find them under 400 used.
good luck 