Have you asked the manufacturer? Not being familiar with the product, I would say to look in the instructions, or ask MTH, or take the cover off and see if there's a volume control inside. If not, it's easy to reduce the volume with a couple of resistors or a potentiometer.
There are things to ask MTH, if there's no built-in way to reduce the volume. You need to ask what impedance the speaker is, for starters. I'm sure that they have an engineer at their HQ who can give you the answers.