What used speaker would you buy with $20,000-$35,000 and why?

For anyone liking the Linkwitz speakers, but wanting either planars instead, or a system with less complexity and channels of amplification, here is an alternative:

The coolest part of the Linkwitz’s are their Open Baffle/Dipole woofers. The other drivers are just run-of-the-mill cones on an OB baffle, nothing special. They create a point source, not a line source as do planars. If you prefer point source, that’s great. But you need three stereo power amps for the upper drivers, plus one for the woofers!

Another way to go is to get open baffle sound for the mids and highs from a planar of your choice (electrostatic, magnetic-planar, ribbon), which will also give you a line source if that is your preference. For the bass, nothing beats the design found in the Linkwitz, an OB/Dipole. Gradient offered a similar woofer for the QUAD ESL63 in the 80’s. There just happens to be a sub/woofer available that is similar but superior to both the Linkwitz and Gradient---the GR Research/Rythmik Direct Servo Feedback OB/Dipole Subwoofer.

The woofer system is available from both GR Research and Rythmik as a kit (the Linkwitz are also offered as kits only) for around $1500/pr. Included are two 12" woofers optimized for OB use, and a plate amp/cross-over with a shelving circuit installed to counteract the acoustic low-end roll-off endemic to OB designs. Plans for the OB/Dipole "H-frames" into which the woofers are installed are available for viewing on the GR website.

The system produces extremely good bass (up to 300Hz) not only because of it’s OB/Dipole design (the Linkwitz do as well), but because of the Rythmik servo-feedback design between the plate amp and drivers. For the frames, you can make your own from the plans, or buy a $500 flat pack that is being made by a woodworker who is a GR Research AudioCircle Forum regular. The frames are very easy to assemble (not much more difficult than an Ikea bookcase), requiring only wood glue and clamps. I'm sure a cabinet shop will put them together for you for an hour or two of labor. He makes the H-frames from the plans, using 1.5" MDF for the side panels/top/bottom, and 1.0" for the baffle and braces! Far superior (less resonant, stiffer) to the H-frame of the Linkwitz. Check them out!

If you decide on planars, you have some great choices. As others have said, you should if possible hear the Sound Lab ESL's. Also the Sanders ESL's, and a new entry--- the ESL with direct drive (no transformer in the speaker) tube amp by Music Reference. MR owner/designer Roger Modjeski designed a similar system for the legendary Beveridge ESL in the 1980's. Factor in a round-trip flight to Berkeley, CA to hear them!

If you have the room for the ESLs above, you have to hear (if you haven't already) the big Magneplanars. Many consider the 3.7i the highest value speaker available, and the bigger 20.7 a great one as well. Be forewarned---they require as much amp power (current---tube amps need not apply) as you can afford. A minimum of 200 beefy watts per for a small room, more for a bigger one. If you have a REAL big room, use a pair of Tympani IV's as (sub)woofers for them!