Adding a Lowpass filter to an amp, how?

[d0gz_bollox]

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?

[Joycey]

The best way would be to filter the high end out before the amp. You can buy an Adjustable Subwoofer Crossover from most car audio places. These usually are powered, and allow you to boost the bass and change the cut off point.

Tried To find one on line this was the best I Could Do

[Joekont]

Joycey's right.
I had an adjustable low pass filter that worked before the amp.
It allows you to change the cut off points and boost the bass etc.
It wasn't a flash make and it cost about £25 from Tandy I think. needs power and a triger but you can take those off the amp.

Got nicked out of a mates GTE otherwise I could give you more info.

[Orbital]

I picked up one for my old amp coz it had no filters on went before amp and powered. got it from cash conveters for £5 bargin and sold it for £15