Thiel 3.6, what will drive them best?

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

One thought,
The McCormack DNA 1 is a fine amplifier that has stood the test of time.
It's WAY better than a number of the recommended amps with the Thiels and can be found on Audiogon for 900-1100.
A good choice that has been a proven match for the 3.6's

You'll be happy with the McCormack DNA1 or the Classe 201.
I'd negotiate the best price you can and you'll be happy with the resulting performance of the 3.6's

You see...the last post (Stevecham) is the exact point a number of us have been making.
Even an amp as fine as the bryston 7bst is unable to deliver enough current for the Thiels.
A high current delivery amplifier is an absolute must.

I have direct experience with the DNA 1 and Thiels in a large room.
Unless you drive them to SILLY loud levels, it will be fine.

You need current and watts... I think 100 wpc is under powering the 3.6's