I can now get the def tech 8060st, cs8060hd center and A60 atmos speakers for the same price as just the svs towers and center. Hmmm.. what to do.
So far we don't know your criteria apart from the budget - if you provide details like room size, listening preferences (movies, music), aesthetical preferences, and anything else, we can recommend better systems than maybe just the svs and DefTech you are comparing. Since you also talk about adding a subwoofer, it is better to specify space constraints too. Many times, bookshelves with a quality subwoofer provide much better sound quality than tower mains and a subwoofer.
With experience I can tell you the other factors are equally important, along with the price, since an ht system is a long term investment for most.
And yes, DefTech or svs, a quality subwoofer will certainly enhance the overall experience.