software

Well i connected my peice o crap rp100 pedal to my Behringer GX212's SLAVE IN so i could bypass the pre and see how it would sound, but i dont have any volume control with the master. The SLAVE IN is stereo and i dont have a y-splitter cable. Could this be the problem? i tried left and right inputs. The rp100 sounded better though btw
The amp is working perfectly besides this