Do I need a sub woofer to match my ATC SCM11s

No doubt in most any room with most any smaller monitor speaker, no matter how good,  adding a sub or two and integrating it well betters the experience by extending the low end that is largely missing otherwise.

If you do it right you should as good off or probably even better off than if you simply invested in a pair of good quality large heavy expensive full range speakers.

My limited audition with passive ATC home monitors (SCM 19s?) at last year’s Capital Audio Fest also left me with the initial impression of sounding "thin" in comparison to other similar rooms there. It left me wanting another audition to get my arms around those particular speakers better in that they appear to be of very high quality in terms of construction and design. It’s one of those audio mysteries to me at present that my curiosity won’t let go.