well oscopes get expensive. you might be able to rent one from a autozone or something.
your local parts store.
if you turn your sub level down to the lowest before your subs shut off. you should be able to turn your gain up to somewhere around 3/4.
i would deffinitly get an oscope and read how to use one and test your stuff.
im only 17 and only have a few years of experience so i wouldnt rely on me haha.