Adrian, it's something that's impossible to precisely diagnose at long range, of course, but what you "perceive" as to the bass level may be normal. Bass frequencies(and to a lesser extent, treble)are less prominent at lower listening levels because of the nature of human hearing.
You should, of course, double check to make sure that the speakers are connected in the proper polarity and that all the bass drivers are in fact operating(including verifying that the connecting strips on the speaker terminals are tight).
