I'm an ATC dealer and long time ATC owner. IMO, you would certainly run them full range. They don't distort at LF, they just roll off. Also, the amp pack has 200W devoted to just an individual woofer so it's not like they are power starved for the low end. They will go lower than you expect if you're judging by the specs. Fair chance you won't want the subs (especially if 100As) but you can try that out before selling your subs.
I personally doubt the Directstream will do the job you want if used direct and comparing to your existing AR pre...but you can try. A select few DACs, inc