As things stand, you'll either add that "bass unit" (i.e. not a subwoof) -- or you'll have to purchase new, fullish range speakers & a new amp. Indeed, if you keep the speakers you'll probably need a higher powered amp anyway (unfortunately): that's judging from your quick description up top.
A side clarification for our Seas afficionado: Seas happens to be in Norway -- not Denmark.
A side clarification for our Seas afficionado: Seas happens to be in Norway -- not Denmark.