Subwoofer suggestions for thiel 1.6?

If I was in your position I would take the simplier route of moving up the ladder and getting a Thiel that has lower frequency response. It would be easier than trying to integrate the speakers and dealing with all the extra wiring. Just my two cents.

I had considered REL, Vandersteen, Martin Logan, and Thiel's very own SW1, but ultimately I ended up with a Velodyne DD-12. My dealer recommended them straight off the bat, but being skeptical (and having heard some lower end Velodynes), I kept looking. After auditioning one with my Thiel CS3.6's (which do not need a sub in any room without serious bass issues, which my listening room, unfortunately, has in spades), I was sold. This has been the most significant improvement that I have made to my system, and trust me, I made many more fiscally significant upgrades, but none made the level of sonic improvement that the DD-12 has. The Digital Drive series line of subs are highly configurable, and can really interface well with almost any room or system. For the 1.6's, a DD-10 might do you, depending on the room size.

The Vandersteen 2Wq sub is one of the best values in subs on the market, selling new for about $1300,and available on the used market for $800-900. It features adjustable "Q" (damping), as well as adjustable sensitivity, so it will match well with most dynamic speakers. Thiel speakers are, like Vandersteens's, both time-coherent and phase-accurate, so the Vandy 2Wq should work well with them. If you intend to use the sub mainly for music reproduction, the 2Wq is your best bet, but if you will be using the sub mainly for home theater, then you might want to buy the Vandy V2W, which has 6db higher output than the 2Wq. As most of the 2Wq owners who post on this site will agree, a stereo pair of 2Wq's is much preferable to a single unit.

The other moderately priced subs that are worthy of consideration are those made by Paradigm, PSB, Hsu, and REL (the Stadium and the Q201E).