Bass knob?

[BC4Lyphe]

I have an Infinity Reference 611a and need bass knob for it. My hu has Front Out and Rear/Sub Out. I need to run rcas from my front out and rear out to a 4 channel amp and then from that amps rca out into my monoblock. This doesnt let me use the hu to control the sub output thus the reason for the bass knob. Is there a better way of doing it? Anyways Im wondering if any phone cord knob will work or if only certain brands. Or if anyone knows the impedance of the RC100 knob from infinity I could just make one. Thanks for any help.