Get someone with a scope - doesn't need to be anything mental. Play test tones from a downloaded test tone compilation. turn the head unit up as high as it'll go without the sine wave distorting and then back it off by 3%, do the same with the amp. This way you can go as loud as you like and shouldn't blow anything. Stay away from EQ and loudness, make sure you deaden the doors and boot at the very least. I am lucky enough to have a Fluke 345 to play with, most sparkies should have access to a O scope.