Do I need a sub woofer to match my ATC SCM11s

JL E110 will do the job and it will certainly enhance your experience. I would not go with anything ported or low cost as it simply will ruin the bass response and lower mid range of your speaker.

Thanks for the suggestion. Has to be that big and expensive? Does it make more sense to sell the ATCs and get floor standing speakers? Any suggestions of good speakers that work well with the Hegel H160? I'm look for a wider and deeper soundstage. Right now it sounds too thin to me, if that makes sense?  I'm open to either solution. 

JL D108 would be another option. Unfortunately great subwoofers aren’t cheap. 

A subwoofer is not a panacea to that thin sound that you describe. You should check out a Spendor floor stander. They have a footprint the same size as a stand mount.
https://store.acousticsounds.com/index.cfm?get=results&searchtext=spendor