looking for specs on potentiometers

JL sells what you are looking for, they are usually around 40 dollars including the cable.  If you insist on making something yourself, you can build an inline (with the RCA cable that feeds the amp) You will need a 100K stereo pot from Radio Shack.  Part number 271-1732.  I have a picture posted somewhere here.  I will look for the diagram.

Using 100K will give you finer control VS using 10K and also allow more attenuation of the signal.  Also, and perhaps more importantly, the pot will provide a component of input impedance to the preamp that is driving it, and you want input impedance to be high.  100K input impedance is preferable and provides a more stable and less noisy system than 10K input impedance.

If your amp has remote bass control capability built in, purchase the correct interface and use that instead of trying to rig some solution yourself.

I have been wanting to do this also, except the amp is a US Amp USA-150 (1999) with the "RBG"- two pinout.

What pot should I get (the board has two 20k for the gain and four 20k for the sub freq.) and does anyone know what each pinout is? I have never seen a pot with only two connections...is one pin not used on a three pin pot? Thanks for any advice/info as these are almost impossible find and can't be that hard to make with the correct knowledge.

It is very common for manufacturers to design the circuit to use a variable resistor. (Only use 2 legs of a potentiometer)