Have never heard of ATC speakers/subs before. So I can't comment on their quality/sound or anything. If you feel they aren't getting the job done or you want to try something else. I say do it! I'm all about switching speaker brands.
Brands to look at, RBH, Focal, Perlisten, Paradigm, KEF and Martin Logan to name a few. If you feel like changing your subs, check out SVS, HSU, Rythmik or the speakers company's listed above have solid subs as well.
For upgrading your 5.1.4 look to have a decent budget of around $6k-$10k. All depends on which models you go with.