coustic 240se with low pass not working

[dirtyfeet]

Hello everyone and thanks for reading this.

I'm having a problem with my car amplifier. Recently the bass started to cut in and out while playing. The amp is the coustic 240se model. I'm just running a kenwood 10" off of it bridged.
O.k. so i did a quick test of the amp's functionality. The amp operates normally in all pass mode and when run in high pass mode it works and the cutoff frequency is adjustable.

So the amp works no problem except for the low pass setting. I also checked power and ground on all IC's and all have power. So this is leaving me to think the problem is before any amplification occurs. maybe the selection switch even. I've looked for bad solder joints and loose caps on the board but nothing looks wrong/bad so far.
All while using an old '70's radio shack analog multimeter.

If anyone has any suggestions or solutions feel free to respond.

Sidenote:
This is what I currently have in my car:
pioneer 4x6's in front door panels
nakamichi 5-1/2's in rear
kenwood deck 4x45W
kenwood 10" sub
coustic 240se amp

 

[dirtyfeet]

this is an update to my problem. so far I have determined that the switch works normally, but havent located where the problem is yet. but it is somewhere in the filter network. maybe there's a bad chip. there's three tl074 chips. im still drawing out the circuit