Thiel 3.6, what will drive them best?

something with lots of power

Look for a used Classe DR-9 I can't think of a better amp built and sound wise and right in your budget. I would also think that a used pair of B&KM200 mono amps can be had for around 500.00 or less and are wonderful.

I used to play mine with a very updated Audio Research VT-60, which probably did not control the bass like a good SS amp would.

At the sub $1k mark, I suggest something like a Belles 150A Hotrod, OCM 200 or 500. Some of the updated Hafler 500 amps sound very good too. You can find all of those amps at less than $1k. Another line of amps to consider are the older Coda amps. Sometimes you can find a nice Coda 10 or 10.5 in the price range you are looking for. They are very sweet sounding and can nearly drive a short, so the Thiels impedance issues should not cause them to sweat. I am not a big Krell fan because of all the inherent buzzing ground loops that they seem to have had each time I owned one, however I have heard good things about the Krell KST-100. Some have said it is more musical than a lot of the KSA's and it is Class A/B so it won't eat itself like the KSA's can do.

I hope this helps.

Unfortunatley, I think the sub $1000 budget is tough for the 3.6's.
As a long term Thiel owner, ther's no substitute for adequate current. The quality output devices and power supplies to provide adequate current aren't cheap to build. I'm afraid you'll be disappointed with amps that don't perform the obligatory tasks in that large room.
I'd recommend the perenial great match for Thiels of this vintage.....Classe amps. The are well built, provide lots of current and sonicaly are a great match for the Thiels.

In this situation, cheap is expensive because you'll have to replace an inadequate amplifier. Happliy, Good Classe amps are generally available, and versions prior to the current line-up can be had at fair prices.
Generally, if you speak to the customer service folks at Thiel, they'll advise against the matching of Krell and Thiels of this vintage. Good Luck

Tvad, interesting reviews that I haven't seen previously. One comment I find curious

aging B&K M200 mono amplifiers, rated at 200 watts each but certainly not current powerhouses.

. B&K rates them as 150 amps peak-peak, which is the basis for my statement. The biggest challenge I've presented them is Totem Mani-2s, which they drive very nicely, and I've no other basis for supporting refuting the rating. I don't presume B&K's rating is necessarily accurate, but it's has me wondering what the reviewer's baseline is causing him to make this statement.

Another amp worth considering in this price range is the Parasound HCA2200. Either the mkI or mkII can be found used in the $600-800 range. I have the mkI which I liked a lot, though I replaced it with a pair of M200's which I preferred. I think the differences are in the personal preference range and some my like the HCA2200 just as well as or better.