just start with the Roland's ourpur volume low, working up to get it to behave about like any other souce device, and you shouldn't have any problem. The Roland's output levels with the volume up may be much higher than what the receiver expects to see. You'll get distortion, but I doubt you could hurt the receiver. Not hard to find quarter inch to rca cables, either. Guitar Center'll have them.
Be careful wth overall volume. In the back of my mind, from some juvenile experience, I have it in my head that the relatively uncompressed dynamics of a piano sound for example can exceed what the speakers can handle even though average levels are not all that high (the initial transient of some instruments can greatly exceed 10 dB 10 times the power of the sustain that follows a twentieth of a second later). I think I killed some stereo speakers with my old school lowry transistor organ once upon a time. If I'm nuts, somebody will jump in, doubtless.