you make a very good pointSurely wattage is fairly irrelevant? I thought that was just how much power it could handle before it went "pop!". What determines output volume isn't the wattage, it's the gain! A 20W per channel amp can sometimes be "louder" than a 50W per channel amp when fed with the same input signal, but the 50W per channel amp will handle more before you start hitting the limit of the components.
I would say your new channel doesn't have more power, it probably has more gain, in which case you may be able to just bias that with a resistor or two and match the channels.