while its true that you can put together a stunning headphone package for cheap, IMO...it never provides that "i'm being pummeled by a big PA" feeling that I really enjoy w/my recordings.
for that, you need speakers. and the bigger the better.
If I were putting together a system on the cheap, and buying everything used...I'd have a hard time looking past a Music Hall Mambo or Maven. its got it all. sweet sound, nice DAC built in...just add speakers and a source and you're done.
a nice pair of monitors (like those b&w's mentioned) , or something from Epos, Von Schweikert, PSB..etc and I'm sure you'd be able to hear what you think you're missing.
I'd also check out cambridge audio gear. I like what ive heard, and they are not expensive (new or used).
audiogon.com !!!!!!!
you just need to know what you're looking for before you go there as the world of audio is massive, and easy to get lost in the forest.
fwiw, I say you can rival the $1k headphone rig in a "real" stereo w/speakers. It'll just take a little luck finding the used gear.
http://cls.audiogon.com/cgi-bin/cls.pl?intatran&1216320159 $800 (I paid less for mine, more like 600)
Lots of nice Epos monitors for sale:
http://cgi.audiogon.com/cgi-bin/srch_fs.plbetween these two options here, you could build a bangin' system for $1500.