Do I need a sub woofer to match my ATC SCM11s

Jeres8860,

     Congratulations on your new REL sub.  
     If you're just going to utilize a single sub in your room, the best you can hope to attain is good bass response and integration with your main speakers at your preferred listening seat.  
     Because bass sound waves are so long, often longer than any dimension in smaller rooms, the placement of the sub within the room is very important for optimizing bass performance at your 'sweet spot' listening seat.  Here's the best method I'm aware of for properly positioning a single sub:
1. Hook-up your sub and place it at your listening seat.  You should  move your ATC mains spkrs and stands about 3-4 feet away from the front wall and into the room.

2. Play some music with good and repetitive bass content.

3. Starting at the right-front-corner of your room, walk slowly along the walls of your entire room counter-clockwise until you discover an exact spot at which the bass sounds the best to you.

4. Re-position the sub from your listening position to the exact spot that the bass sounded best to you.

5. Sit at your listening seat and verify the bass sounds good to you.

6.  Re-position your left and right ATC speakers on their stands so that the midrange, treble and sound stage illusion are optimized at your listening seat.  

7.  Adjust the volume and cut-off frequency on your REL sub until the total musical presentation sounds best to you from your listening seat.  It's easiest if you sit at the sweet spot and have someone else perform the adjustments at your direction.

     You'll probably quickly notice a few things.  Positioning the sub in a corner will reinforce the bass and cause it to sound exaggerated and boomy.  In my experience, avoiding corners and positioning the sub, even a small distance, away from the nearest wall results in a more natural and well integrated bass response.
     I've learned to treat my system as two systems; a bass system and a mid/treble sound stage illusion (imaging) system.  I've attained the best total system sound in my rooms by positioning my sub(s) first without my main speakers even connected or in the same room.  
    Once the solid foundation of good bass response is installed and present in the room and system, it's really just a matter of inserting or overlaying good mid,treble and imaging response into the room by properly positioning the l+r main spkrs in relation to the listening seat that remains to achieve very good overall system response.  I suggest best results will likely require your ATC on stands to be positioned a significant distance away from your front wall and into the room (4 plus feet if you're able).  You may also benefit in your room by experimenting with placing absorbent or diffusing wall treatment panels at 1st reflection points along each side wall.
     I believe treating your system as 2 systems is useful because the bass sound waves in any given room actually behave differently than the higher frequency sound waves.  Long bass sound waves are much more difficult to control via room treatments in a given room than the much shorter and more directional mid and treble sound waves.  I'd suggest adding additional subs to improve in-room bass response before adding large and expensive bass room treatments.
     These very different bass and mid/treble sound waves also do not interact, influence, depend on or affect one another.  As I understand them, the only relevant way these sound waves are related at all is that some fundamental bass tones reproduced by a sub or subs may have upper harmonics that extend into the range of directional frequencies that may be reproduced by the main speakers.  This is not really an issue since the higher frequency bass harmonics reproduced by the main speakers actually assists our ears/brains in properly positioning deep bass instruments and voices within the soundstage illusion.

Tim  

Thanks so much Tim! This is super helpful. Given my space, I'm not sure I can achieve all of this, but will definitely try and use as much of your info as possible. The Rel is supposed to come in this week, can't wait to start experimenting. Thanks again! 

Hello jeres8860,

     You're welcome, I'm glad you thought it was useful.  

    I don't consider myself a bass expert, I've just learned a lot over the years trying to integrate good bass response with various Magnepan speakers in my system and room.  I've tried a single sub, 2 subs and the 4 sub AK DEBRA dba in my system and have learned that 2 subs will provide better bass response and integration than a single sub.  I've never tried 3 subs but believe it would work better than 2 subs if positioned properly and I know that a 4 sub dba will provide state of the art in-room bass response and integration in virtually  any room and with any speakers.  

     I know of no single sub, no matter the quality or price, that is capable of providing anywhere near the sota bass performance of a 4 sub dba.  However, I know you'll be able to achieve very good bass performance with your single REL sub if you set it up properly. 
     I think the most difficult and subtlest steps for you will probably  be setting the REL's volume and cutoff frequency optimally.  I'd suggest setting both as low as possible while still sounding natural, well integrated and the best to you.
     New sub users often make the mistake of setting both too high out of a desire to make the affects of their new and likely expensive sub clearly and obviously heard. Resisting this temptation will result in an overall sound affect that is more subtle but more natural and better integrated with your main speakers.  Remember, the goal is to extend the bass definition and dynamics of your system's sound stage in a seamless manner so that it sounds like a cohesive whole.
     Please let us know the results once you have it properly installed in your system.

Tim