I'd suggest the one with more power.
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From what I know, Thiels are tough on amps. Thresholds don't have a ton of power and really aren't happy below 4 ohms. If you push things really hard, both amps may have issues. The difference between 60 and 100 watts is only about 2db. I'd look at something with a lot more power and lower impedance ratings...just my 2 cents.
S1600's @ 800 Watts per channel, don't have a ton of power?
Threshold did not recommend continous loads below 4 Ohms, but, they burnt each amp in with a 2 Ohm load for 48 hours before testing and delivering. Threshold used Thiel CS 5's which dropped down to 2 Ohms, as one of their reference speakers, and before that they, used other Thiels too. It is reputed that Thiel has used various Threshold amps as reference amps to develop some of their speakers.
Thiel recommended between 100 and 500 Watts for the CS 6, and that the amp should be able to double down into 4 Ohms and at the very least be three times as powerfull into 2 Ohms as it would into 8 Ohms.
Your room and desired listneing levels will determine just how much power you need. I suggest you 1. buy the best quality and 2. the most powerfull amp into low impedances that you can. A good starting point might be something like 800 watts into 2 Ohms.