What did you mean by 'low'? Did you mean iMac speaker is not loud enough? Or not have enough bass (low freq)?
Because in term of loudness, I find built in iMac speakers are loud as hell, I set the volume on 25% and it already loud enough, but harsh and tinny.
If you mean the speakers are not bassy enough, you hit the spot. There are no other solutuons. Get an external speaker! Or even better get separated amp+bookshelf speakers.