Jason:

Radio Shack is a pretty good place to go to pick up the required materials and they undoubtedly have books that cover projects like this. For the level control you would basically need a couple of 10K ohm (this can range from 10K ohm to 50K ohm - it's not critical, but 10K ohm is preferrable) audio taper potentiometers (one for cach channel). Also you will need an aluminum project box to put them in, the input and output RCA jacks, shielded wire, and a couple knobs of your choice. You could purchase a couple interconnects and cut them in two for the input and output wires, so the RCA plugs would not be needed. The important thing to keep in mind with these outboard level controls is to keep the length of the output to amplifier interconnect wire to about a foot or less. This is because if it is too long, the capacitance of the cable will cause a rolloff in high frequencies. If the length is kept to a foot (or less) then there is no danger of this happening.