Instead of cables, get (or build) a passive d/I.
- reasons to get a d/I so you don't blow your instrument levels.
- how to build a passive d/I.
Essentially the passive d/I box can turn your line to mic level and you can do the rest on your audio box.
I use a passive d/i for my active guitar pickups. They're too hot for hi-z (instrument level), so I use a d/I (cheap £20 one) to get them to mic level. My d/I box is made by Stagg and was cheap but effective. It takes the active pickups and dumps them out as both mic level AND instrument level. The hi-z (instrument) then goes to a boss fx rig/guitar amp, while the "dry" mic level (low-z) goes to my audio box.
I hope that helps some. Good luck if you go for the homebrew approach as it doesn't look too difficult to build.