These speakers have a -3db FR bottom at 33 hz. That is pretty low. That presents a couple of problems for using a sub. You would have to (I would anyway) drive the Thiels full range so balance wasn't screwed up (let alone signal degradation) by putting it through a crossover to roll off the bottom end to accommodate a sub. Next I would want a sub that was flat at 20hz or close, think big and expensive, and one which had a sharp roll off starting at about 32hz so the sub didn't extend up into the mid base.
Which leads me to suggest that you insure that you have already maximized the bass potential of the Thiels by careful use of boundary reinforcement (use a SPL meter and test disc) by changing speaker placement AND the listening chair. Both play a large roll in perceived bass.
I've only one been successful once in making a good sonic match with speakers and sub's (I like 2 more than 1) and even then I had to cede that the crossovers did cause the loss of a bit of resolution in the main speakers. It was worth it only because the sub was substantially better for overall bass and the cross over was sharp at 200hz for the subs and gentle for the mains.
Which leads me to suggest that you insure that you have already maximized the bass potential of the Thiels by careful use of boundary reinforcement (use a SPL meter and test disc) by changing speaker placement AND the listening chair. Both play a large roll in perceived bass.
I've only one been successful once in making a good sonic match with speakers and sub's (I like 2 more than 1) and even then I had to cede that the crossovers did cause the loss of a bit of resolution in the main speakers. It was worth it only because the sub was substantially better for overall bass and the cross over was sharp at 200hz for the subs and gentle for the mains.