the one in the link is a ClassD, so about as efficient as they come, but still.
is that the one you were using?
from what i've been testing with loudspeakers, wattage and ohmage definitely don't say everything.
speakers with similar specs will differ widely in efficiency (and therefore volume.)
so trying different speakers is definitely worth it.
on a sidenote, i've also found that oval shaped speakers tend to produce more bass. 🙂