Adding a Lowpass filter to an amp, how?

[adam5]

I have an amp that has four channels, two of which can be set to low pass filter and two which can be set to high pass filter. Or they can be both set to use no filter. I want to convert the two channels which can use high pass filter to low pass. so I can use all four channels for subs (bridging them). Can I buy a low pass filter thing that I can put between the amp speaker out and the sub that'll only let low frequencys through (40hz). Where can I get such a device from and likely cost?

[Stuart]

yes you can get passive low pass filters (take a bit of power but not enough to complain about. or you could get a DSP (digital signal processor) or even an analogue one (its a half din sized graphic eq) and then turn the filers on the amp off and run it like that. then you have presice control over the phasing and level of bass from both bridged channels

"I'm a junky with a monkey"