Subwoofer gains, multimeter

[Squiddler]

So basically, I've worked out the voltage I need my multimeter to read, i'm using a 50 Hz test tone etc, when I connect my multimeter to the positive and negative terminals of my built in amp on my subwoofer, play my test tone and slowly turn up the gain the voltage reading isnt increasing ?! It's really pissing me off, it just sits at a constant 25v if anyone has any idea of WTF is causing this i'd be so greatful, I blew my last sub because the gain was set too high and I dont want to make the same mistake twice,



thanks