See if you can find some Castle Richmond's on the used market. Very full bodied little English monitors that can be had for about $350. There was an Audiogon dealer that had these if i remember correctly. Sean
GR Research Paradox speakers have been raved about on the AudioAsylum (www.audioasylum.com) ... if you assemble and finish them yourself they will cost around $400. Finished price is less than $500 factory direct. They look beautiful and feature good quality drive units. Search on Google or other engines for the GR Reasearch site.
Spica Tc-60 (I own Angelus and the imaging is excellent).
Epos ES-11 (older version of M12) another good imager.
The venerable ls3/5 minimonitor is a good apartment spkr. I'm not sure of the used price in the US.
Assuming you stretch to a good standmount at this stage the subwoofer route (e.g. a used REL) would offer tremendous sound quality if you can escape the apartment in the future.
I'd suggest you consider Maggies, possibly the little MMG's. Let me explain:
For a given sound pressure level at the listening position, a dipole will put 5 dB less bass out into the environment. That means you can listen 5 dB louder before your neighbor starts banging on the ceiling.
In addition, Maggies are utterly boxless and have very nice timbre. You can tailor the top-end by using resistors (supplied). Maggies are voiced warm and forgiving, and while there are other speakers that do a better job of pinpoint imaging, Maggies give you a very good sense of acoustical space (ambience).
Maggies aren't perfect, but they do a very good job with many of the things you value. You didn't mention this, but Maggies have fewer annoying little colorations that remind you you're only listening to speakers. To me, that's a big part of being able to close your eyes teleport to the performance.
Best of luck in your search.
Agree with Seandtaylor99 about the LS3/5a, with the only question being your taste for rock. Also, they tend to go for around $800 here and on ebay (I have 2 pairs of the original Rogers 15-ohm type, so I tend to notice).
I recently picked up a used pair of Spendor S3/5s used for $600, which I think might actually beat the LS3/5as. Slightly warmer sound, soundstage a little deeper and fuller on the bottom. Think you'd like the top. Need stands, as several of the other recommendations do, so have to factor them in to your budget. There's a very favorable review on the EnjoytheMusic.com site.
Spica TC-50 or TC-60 (less desirable). Cheap used; around $250. TC-50 soundstages exremely well; speakers disappear. Top is rolled but not dark sounding. Mids are clean, so it's up to your electronics- use tubes for warmth (Rogue 88 would be excellent with this speaker). A very fast detailed speaker that has a lively sound even at low volume levels. Well suited for apartment listening.
I heard the S38 the other day and was impressed. I have not liked JBL's past speakers, but the S38s are well balanced, pleasant sounding speakers with good bandwidth. I do not know how well they soundstage as they were placed in the top corner of a store. But the midrange was smooth. They also seemed okay in the highs, but, ya' know, i was way off-axis. Also look in to B&W. They have a great soundstage, and good detail. Thier mids are really good, but are dry, not warm. I have not heard many, maybe any, speakers in your range that are particularly rolled off. You may have to address that in your amplification. Maybe the MMGs are better in this department than dynamic speakers, but i have never heard them. Good luck
Some OUTSTANDING recommendations, thus far. I have to agree about the Spica TC-50's. These things throw a soundstage unlike anything in this price range. I'm often tempted to buy a pair. The TC-60's "improved" upon the TC-50's with deeper bass (a minus in your case) and greater dynamic range (higher spl's, again a minus). To my ear the 50's are better sounding. They're extremely attractive, to boot. Finally, because there's such a cult following, there are still parts available in case the neighbors are gone and you get a little carried away. I don't actually own these speakers, but many of my non-audiophile friends own them. About 10 years ago, my friends would ask me to help them put together a "modest" system. I told them I'd be happy to go with them to listen, but that good sound is highly subjective, so it would be up to them to decide what they could live with. One by one, I would take them to the various audio salons in the sf bay area and all but one of them ended up choosing the Spica's. (The other was a bit of a rocker and picked up a pair of Paradigm 7SE's)
I've not had an opportunity to hear the Triangle Titus', but I bet they're terrific. Another gentleman recommended the Magnepan MMG's...another INCREDIBLE speaker for the money. Any one of these three can be found for under $500 (including stands in the case of the Spica or Triangle). Of course, you could actually get a brand new pair of MMG's for only $50 more than your budgeted $500. Plus, they'll hold their value pretty well as Maggie's tend to do.
That's my longwinded .02. The good news is that there's lots of good bargains to be found in that price range. Happy listening and best of luck!