Best $4000-$5000 loudspeaker

I would suggest used Aerial 10T's or Dunlavy SC-IVA's (not the IV's without the A) both of which are under $4k now. Both these speakers need room to breath, though, so your room size is a consideration.

if you can stretch to $6k- used Coincident Total Victory...

How about Merlin VSM-MM's (I found mine for $4500 as demos)or MX's (might be able to find them used for around $5,000-$5,500).

They extend down around 28 with BAM, work well in an average size room, and can be enjoyed at low to moderate levels.

Swklein listed most of the speakers I would have included. As an owner of the Vandersteen 3A Signatures, I can highly recommend them -- they compete very well with speakers costing up to $5000 (MSRP). The one other speaker I'd add to the list is the Magneplanar 3.6's, which are a superb speaker if you have the right room for them, and a fairly powerful amplifier (200 wpc minimum).

For new, I like the Focus Audio FS788 and Amphion Xenon.