Unidentifiable thing under drivers seat
 

i have a 2003 xl.7 and i love it more than anything! ive had it for almost a year now and ive looked in the manual and everything, ive googled this question/problem many times and i just cant seem to find any information on what it might be.
its not in the owners manual, its not on the suzuki website, and every "car expert" i mention this to doesnt know either!

so heres whats up: i have this black box with this knob on the front of it. (it may have other adjusting handles) ive tried not to mess with it too much. anyway, the knob i have found has a "max" and "min" setting.
for a while i had convinced myself it was an amp for the speakers, because the speakers in my xl7 are such great quality. but im just not too sure anymore.

does anyone have any sort of answers!? what is it? this is driving me nuts!

heres what it looks like, the first picture is from the front and the second picture is what it looks like from the behind.
if you can help me, then please do! itd be greatly appreciated (:
thanks, love Hannah!

Better late than never
 

That "thing" is a subwoofer installed in certain SX4 models such as the Touring Edition. Literature will say your SX4 has 9 speakers, but you'll only see 8 visible (two in each door) .....the 9th is that subwoofer and the gain control is like a volumn knob.