T37,
Welcome fellow TL lover!
As Joe D'Appolito states: "Transmission Line loudspeakers have long enjoyed a small, but dedicated following. The advantages of TL's are well known. They are essentially non-resonant enclosures, producing a deep, well-controlled bass response. For a given driver, bass response will extend well below that produced with either a vented or sealed enclosure using the same driver. Above a few hundred Hz, the line filling material completely absorbs the driver back wave, giving the TL an open, non-boxy sound." And like a rear loaded horn, the sloped internal baffle helps to direct that midrange backwave away from coming straight back out through the semi-transparent cone material to reduce smearing.
The Vandy is the classic here, the historic variants are highly available used and at good prices, check audiogon.com. Love the 6 dB/octave crossovers and stepped baffle, but don't like 2 crossovers or the passive radiator.
The Salk and Thor use Augspurger/King design. The trade off I see there is driver quality in the Thor versus the convenience of the Salk being pre-built. I'll always go with quality parts if given the choice, so I voted for the Thor. Madisound is old school in their methods and quality of service. But I'm not a fan of the MTM arrangement as it restricts vertical mid/bass imaging.
I've been listening to TLs for 30 years and currently own Bob Brines FTA-2000. These speakers use "the mighty" Fostex F200A full range driver (8 inch, 90 dB/m/w, 8 ohms, 30 - 20,000 Hz raw) with a big Alnico magnet that Madisound currently sells for $388.50 each. Bob sells speaker plans, panelized kits, assembled/unfinished cabinets, or fully completed speakers. The completed FTA-2000 is in your price range. The big 8 incher will "beam" above 4,000 Hz, but provides complete coherency for nearfield listening (and helps take the room effects out at those higher frequencies). With a single listening position I've not found the beaming to be an issue. Using single drivers means that you're running "active" by default. IMO TL, single driver, and active are the only way to go.
Happy hunting.