Monitor or fullrange ?

If you like the subwoofer, you might want to take a look at the Definitive Technologys Power Monitor series. Pretty good full range in a small box, they will blend wonderfully with that PT15 sub ya got. They are not bi-polar like most of the Deftech speakers are.
Ive heared them at a local audio store and i thought they sounded fantastic. They have 3 different models of the ProMonitors, you could pick up a new set for that budget of the lowest priced, or a set of the higher priced models used for about 500

M2C

For about $600 (just slightly more than your budget) the Platinum Audio Solo loudspeakers are definitely worth considering. This is one great, and formerly expensive, speaker whose used price fell to earth, but with performance still up in the heavens.
If you can afford to move right now, there has been an Audiogon ad for Green Mountain Europas for $650.
Both of these speakers are amazing, and well worth the stretching needed to afford them.

Green Mountain Europa
B&W CDM 1 SE
Paradigm Studio 20 V.3
Monitor Audio Silver S1

I believe any of these would do a fine job.

In that pricepoint you truly must give a listen to the Quad 11/!12L. There is a decent amount of information on these speakers here on the 'Gon but they are the absolute darlings of the British Press (11Ls have won product of the year - the last two years from What Hifi for example)

I've preached about these speakers in many threads :) - But you can dig up the threads for more info. But in addition to their sound, the fit and finish of these speakers is second to none, as my girlfriend will attest to. Quad's Website information on the speakers and their finish, etc. Best of all they should be in your price range, new.