You may just have a problem with a ground loop. You can search prior threads for solutions and discussion. Try a 2 to 3 prong adapt (from any hardware store) to defeat the ground on the Rowland and see if the hum goes away. If so, then you do have a ground loop and should follow the trouble-shooting steps discussed in prior threads to optimize how you get rid of it (i.e. not leave your amp's ground bypassed).
Best of luck,
Jacob
Best of luck,
Jacob