Best subwoofer below 40Hz for Magnepan .7?

The title says it all! I want subwoofer bass below Magnepan's limit of 40 Hz. Magnepan will not recommend any but their own model, which does not extend beyond 40 Hz and are not true subwoofers. What subwoofers will work with my .7s?   Thanks for the personal and meaty responses on this forum!