Assuming you are talking a Kick Drum... 50Hz!
Quite honestly, with a 12" woofer and at 35Hz to 20kHz, +/-3dB, I would not get too excited about a sub. Your speakers were designed as full range music speakers before subs became a common part of the equation.
But be your own judge, here are some frequencies of various instruments:
http://www.independentrecording.net/irn/resources/freqchart/main_display.htm
This is an interactive chart, so drag the cursor over any area of interest.
Aside from pipe organ and modern electronica (which are not addressed on this chart), you are not missing much and I would not recommend concerning yourself with a sub unless you feel what you have is not getting the job done.
To get the full benefits of a sub, you need good bass management, to get good bass management, you will need a receiver designed with a crossover to unload the low frequencies from your speakers, to use that, you need to have a TV connected, etc.