Have you tried contacting the company? They may have some ideas for you. A lot of times a hum or buzz or a simple thump can be an indication of a difference in ground potentials. If you have wide voltage swings you may want to look at getting a .5 Farad Capacitor which is normally used in Car Audio to help provide a little extra umph to the amplifiers during heavy Base. Caps help to stabilize the voltage. These are also used as noise filters in some vehicles as well. Boats can be a problem sometimes with electronics originally designed for automotive use due to the fact the body is normally a non conductive fiberglass or wood and instead have to depend on wires.
You may also have to get a noise isolator between your amp and your PC outputs if you can't figure out another way. These are called Ground Loop Isolators.