A4 Facelift Sound System

I have a question about my father's A4 B5 Avant

There's a speaker in the trunk behind the left side panel, I don't know, don't look!
But it seems to be only responsible for the bass!

Now the question is, is that part of a soun system or what?
But there's no BOSE or anything in the speaker paneling? Is that an audio sound system or is that with Allen inside?

So the sound in the car is peculiarly well you can turn up quite loud but no signs of overload say rush!!

Oh yes and Navi+ he is of meaning is also back there!