Make sure that you have the "Depth - Slam" selector set to depth. Not sure what the slam setting is for, but when selected it sounds too boomy and could possibly do what you are describing.
Help with Setting Up REL Sub
After getting what I think is a satisfactory setting, I often hear a low level vibration, or what sounds like a rapid flapping of the speaker cone, when playing a cd with a female singer. Do I have one of the settings too high or is this just a characteristic of subs? My main speakers are Dynaudios 1.8MKII. Thanks for any help.