Good upgrade amp for my Thiel 3.6s

I had good success driving my 3.6s with Bryston 7B st monoblocks switched to "high current" mode. These are rated at 500 wpc into 8 ohms and 800 wpc into 4 ohms in normal "series mode," and 500 wpc into 2 ohms in high current "parallel mode." The 3.6s demand this high current capability as their impedance dips below 3 ohms at certain frequencies. The 7B st's have been rated A on Stereophile's recommended list. They're solidly built (20 year warranty) so they're probably pretty decent as a second hand purchase. You can get them below your $4k cap here on Audiogon.

I had always heard that Thiel and Levinson are a match. I had been running a Levinson 27 with my old Thiel CS3's. I recently picked up a pair of 3.6's and, with the 27 (100 watts), there seemed to be something lacking.
I just (literally yesterday) added a Levinson 333 (300 watts) to the 3.6's and it made a HUGE difference. With sensitivity at 83db, they need POWER!
Now, they are remarkably improved -- soundstage, detail, both in the highs and the bass, clarity, etc. If you don't mind buying used, like I did, the 333 is in your price range. You could probably even find one, like mine, still under warranty.
I had been looking for a 332 (200 watts) and had even asked Wes Phillips by e-mail what he thought would be the way to go, including some options for tube monos, like the VTL's. He thought the 332 with the Thiels would be, in his word, "super." I was lucky enough to find a 333 someone was selling, used, for the price of a used 332 and jumped on it. I called Madrigal, first, and they gave me the "in service" date for warranty purposes and checked to see if there was any history on it.
I highly recommend the Levinson, but, whichever way you go, definitely MORE POWER!

I would think that you'd want an amp that was not inherently bright sounding as the Thiel 3.6s can be very revealing. I can highly recommend the McCormack DNA2 or DLX or any of the SMc Revisions. The DNA2 puts out 300/600/1200 wpc into 8,4, and 2 Ohms, and these are probably conservative ratings. The DNA2 amps are "giant killers" well beyond their original $4300-5000. MSRPs. The DNA2s can be found used in the $2400.-$2800. range. I use a DNA2DX Rev. A with Vand. 5 speakers-- the 5s don't need all that power, but your 3.6s could use it.

BTW, McCormack Audio now has the DNA-500 out (that's 500 wpc 8 Ohms), but I've yet to see it advertised used. Peter Moncrief of IAR declared it "the best amplifier on the planet"-- $6700. new I think. Good Luck and Cheers. Craig