i think, with $500 budget, you able to get for new and nowdays speaker type. second hand speaker is ok, but it's depend on how the old pal usage. because i believe the characteristic of internal material will change especially moving parts.
I have NAD c3020 matched with B&W DM602s3 sounded great Should be try with your 500$
you should think greatly of my recomendations because i have owned both.. paradigm,,say studio 20 used ,,around 400 or a little more,ones more detailed and full.. http://cls.audiogon.com/cgi-bin/cls.pl?spkrfull&1129630923 or one of my favs ,,linn kans less then 400,,offer ,,very fast fun and musically exciting,, http://cls.audiogon.com/cgi-bin/cls.pl?spkrmoni&1131641400