f=1/(2*PI*R*C)

rc filter would work, OP-AMP have it's advantages as well. Both will use the same eternal formula.

http://en.wikipedia.org/wiki/Low-pass_filter
http://www.discovercircuits.com/PDF-...wPassFilt1.pdf

Easier than calculate the ultimate enclosure for a x
Subwoofer in a dipole setup.