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.

It does sound like you are over driving the sub. What model REL do you have, how big is your room and where is it physically positioned in your listening room? The physical position of the sub will make a difference in how hard the sub has to work. Also the position of your Dyaudios and seating position will change the bass output of you mains where you may be able to get more bottom end out of them allowing you to turn down the REL. When I mention moving the mains and seating position, I'm not suggesting that you will have to move your entire room, just a few inches can make a ton of difference in both. In positioning the sub, try corner loading it or moving it out six inches to a foot from your wall. I think most people feel that the sub should be up against wall to get it out of the way. You have it give it some room to work. Unless you have a monsterous heavily damped room with concrete floors, you should be able to solve your problem by just make a few changes. Just my two cents. Good Luck..
Yes, sounds to me like you are over working the sub also. I agree with positioning too. A REL is not like a typical subwoofer, so forget about what you've been told or learned about subwoofers, and go read this link at the REL website.
(Actually print it out).
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.