Yeah, $600 can easily get you a professional, used rig. I picked up a GK 1001RB-II and 410RBH for $700 about two years ago locally before I even joined talkbass. I've seen classifieds here where you can pick up a nice GK, Hartke, Trace Elliot, SWR etc. head and a Neo cabinet for $600 total (plus you're supporting this awesome community).
GK 700RB-II or 1001RB-II or 800RB with a GK Neo (212/410) cabinet
Hartke HA3500 or LH500 with a HyDrive 410
anything and everything Trace Elliot
If you're only looking for a head I'd try and hold out for a used Peavey VB
-2 or Traynor tube head...