I don't think I hear my subwoofer working when I'm just listening to CDs. Heck I don't think its even on. Is there problem or its only use when watching movies?
If you have your fronts set to large, many AVR's will not output to the sub. Some for example H/K units have more robust bass management and can be configured around this.
The setup option for bass management usually go something like this....
Speakers set to large = LFE only to the sub, the LFE channel is a descreet channel contained in DD 5.1 and DTS sources.
Speakers set to small = LFE + anything that is below the cross over point you chose for your sub.
Some AVR's offer speakers set to large and LFE + Any content below the a chosen cross over point being sent to the sub. (This is generally not a good idea because your speakers and sub will be duplicating content which can lead to cancellation issues)
Generally if your speakers are set to small, the sub will run in stereo mode.
Now some AVR's offer a Pure Direct Stereo mode, and will not send any content to the sub when in that mode, while others stillmay send content to the sub in that mode.
Obviously, much of how this will work will depend on you particular AVR and the Bass management offered, and also much of it can depend on how you have bass management configured for your particular AVR or Pro/Pro being used.