I am using Decibel Audio Player 1.02, on Ubuntu 9.04. (I don't know what gstreamer version DAP uses). I first filed this bug with Decibel Audio Player on Launchpad, and was directed to file it here instead. Because I am using gstreamer with Decibel Audio Player, the behaviour and replication descriptions are necessarily DAP-specific.

The following behaviour applies both to regular mode, and to gapless mode using the "--playbin2" switch. Also, it applies to FLAC files, but NOT to MP3 files, or at least not to the ones I tested.

When I am playing a FLAC file, (one which plays in other players with no problem), activating the equalizer and using the low-frequency sliders to adjust the frequency response results in a 'rumbling' type of noise or distortion. Additionally, using the high-frequency sliders may or may not result in extreme distortion that sounds like an amplifier in severe clipping. (The latter may occur only after the player has been open for whle, but I haven't fully characterised this behaviour). Both of these behaviours occur regardless of whether the sliders are used to increase or decrease the amplitude. In both cases the distortion goes away upon clicking the 'center' button on the equalizer. (I have also experienced program crashes, wherein DAP stops playing and becomes completely unresponsive to user input, while using the equalizer).

To replicate, make sure the Equalizer plugin is enabled, then open the equalizer and click on the 'Center' button to ensure that the equalizer is effectively off. Play a FLAC file, preferably one with a vocal and minimal instrumentation, (I used 'Beautiful Goodbye' by Amanda Marshall), and observe that there are no unexpected distortions or artifacts. Then, adjust the 29 Hz slider in the equalizer, and note distortion not previously present. (It may not be immediately audible, so listen for at least 30 seconds if you don't hear it immediately). After hearing the distortion, click on the 'Center' button again, and note that the distortion disappears. Then, try other sliders to observe any unexpected effects they have.

There is another Equalizer bug that may be related to this one, but its symptoms are different so I'm putting it into a separate bug report.