I used to own Triangle Titus' which, like your speakers, don't go below 60hz. I got a REL Strata III subwoofer for bass augmentation. I dialed the cutoff to around 50hz and I got very good results (you have to play a lot with placement though).
There are some pretty good value subs out there like the HSU VTF-2. Try it first and if you don't like it, return it.
There are some pretty good value subs out there like the HSU VTF-2. Try it first and if you don't like it, return it.