Thiel 3.6, what will drive them best?

Unfortunately IMO you will not find anything in that price range to drive them to their full potential as you put it. Thiels really need both high power and high quality power to be great. If budget is an issue then quality over quantity is better. But in your price range either one is going to be very difficult to achieve.

Honestly, I think you have a big dog that you can't afford to feed. Sure, you can keep the dog alive, but he'll never be all that he's capable of.
I think both the B&K M 200's and the Classe' amps will adequately keep you going. I ran the M200's on older easier to drive Thiels with no problem, but the 3.6's are a much tougher load. I had a friend run Dunblavy SCIV's which while still easier than the 3.6's are similar. It was amazing how much that system improved when he bi-amped with an additonal factory matched pair of M200's. Though he ran all 4 factory matched amps through the Dunlavy's pasive cross-overs, the sound opened up dramticaly when using 4 as oppossed to 2. Much more than I would have ever anticipated. Draw your own conclusions. If were in your shoes with Thiel 3.6's, I'd try to figure out a way to purchase the biggest Krell I could afford or find an easier load.

I just went through the same search in a similar budget. I ended up buying a pair of Quicksilver Silver Monos, but one got lost in shipping (a long, sad story for another time), so I never got to hear them. Bummer.

As a "quick fix" (until I can afford what I want) I bought a used Aragon 8002 for under $500. It sounds very nice, with good bass control and more than enough headroom. It's rated at 125 wpc into 8 ohms and twice that into 4. Hard to imagine a better sounding amp at that price. You may also consider an Odyssey Stratos in the used market. Good luck.

aragon 8002!!

You can consider the used Parasound AMP with 200W plus to drive your 3.6. They may pop up on agon from time to time in about $1000 range.