Adjustable crossover and level control

Is there any good source for an adjustable low pass filter and level control for subwoofer? I bought one of the little boards on eBay with a frequency and volume pots and running rew on the sub confirmed that it wasn't adjusting the cutoff point. It still went to just as high of a frequency and just dropped the low bass some and the overall level when adjusting the crossover point.
I have an icepower amp that I'd like to use to power two boxes but I need an option to control the inputs to it.
Any suggestions?