BR-1600 --Synth Noise

[Envy The Pea]

Greetings. I own the BR-1600. Haven't had it a long time but I really like it. Still a lot to learn on programming the drums...

My problem right now is unwanted noise. I have a Roland RS-50 synth connected to inputs 7 & 8. I can't seem to get a decent volume level without also having a significant amount of background hiss. What the heck's causing it? I'm connected with a couple of ( I thought) pretty good guitar cables. My keyboard sits right next to the 1600 (my wife won't let me spread out much more!)... could that be part of it?

Appreciate any suggestions that will resolve this (the noise-- not the wife!)