The sub primarily is for LFE (low frequency effects). That is why you set a crossover point so anything below that point for the other speakers gets sent to the sub. All the speakers have a different frequency range. 80hz is normally a good starting point, and meets THX recommendations. Some movies are better than others and will blow you away.
In regards to CD's, it really depends on the recording. The m60's do a great job in the bass category, but depending on the CD you have, there will be some LOW's coming from the sub. I have a few music DVD's that you can really hear the sub. One example is Diana Krall's Live in Paris or Live at the Montreal Jaxx Festival. The stand up bass just rocks my world.
Another issue could be your sub placement. Do you have a picture or drawing you could share with us of your setup?
