Do I need a sub woofer to match my ATC SCM11s

It’s very difficult to integrate sub to the system and most of the subs are awful.
The ATC SCM 11 are very good speakers you sure that you are lacking low frequency in your system ? If so I would try to upgrade all the cables including the power chords and if it’s not help only than moving to floorstanders .
I have had before sub in my system and got reed of it because the low frequencies were artificial and exaggerated.

Upmarket power chords can make huge difference also interconnects and speaker cables but you have to match them carefully to your system ,if it doesn't help and you feel that your speakers are not good enough in low frequencies it's better to upgrade to floorstanders than adding a sub to your system, I in general not a big fan of standmount speakers they can be very good at transparency, image and clarity but lack in dynamics and low frequencies.

If you want really professional sub I can recommend the ATC :

http://atcloudspeakers.co.uk/hi-fi/loudspeakers/subwoofers/c1-sub/

Regarding power cables I am using Atlas cables but they are quite expensive .Wireworld and Audioquest have very good power cables in reasonable price.

Good luck with your new sub !
Please keep us update how did they fit into your system and if they fulfill your your expectations.