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.

You're right about risking damage at 500 watts. Damage can occur at lower wattage, too. For example, the tweeter will fry if you put 400 watts into it. Also, if your amp power is relatively low and you turn up the volume until it clips, the clipped signal can damage your speakers fairly easily. I generally regard the manufacturer's upside wattage specification as a guideline not to turn the volume beyond a certain level. The 400 watt specification suggests to me that the speaker is relatively robust and can play fairly loud. I believe the speaker can withstand substantially higher power peaks for very brief periods using musical material.

The low end specification of 100 watts suggests that the speaker is relatively inefficient, which the 86 db sensitivity rating implies also. Of course, you can get decent volume out of the speaker using only a few watts, but you need more to reproduce peaks at realistic concert hall levels. I don't know if the 86 db rating is on target or too high. I would rely on Thiel's honesty in ratings and engineering capability for properly identifying the speaker's sensitivity. Naturally, the loudness we perceive will be dependant on our room acoustics, which could easily cause as much as several db of difference.

I never had any trouble driving my 3.6's with the 7B st's. I never damaged any of my drivers. (I did once overtighten the speaker's cable binding posts, to my huge embarrassment.) However, I rarely turned the volume way up. Mostly, I listened to small acoustic jazz groups at live volume levels. I think the 7B st's are a good match for the 3.6's for their high current and high power capabilities, as well as good sound and reasonable cost.