You should just get an amp, and a 5.1 bundle.
for $400 , you can get a complete 7.1 system from onkyo.
I wouldnt go with the audioengine 5 unless your a musician who wants studio-monitor like bookshelfs with hardly much bass on the low-end.
Note that treble detail on the A5 isnt that great compared to other speakers, however it is much better than bose or apple's speakers and possibly klipsh's
But there is no way to connect a sub, as there is no crossover. Unless you want to buy a subwoofer with inputs and outputs and connect it to the a5.
There are many options you can choose from, from a svs system to a onkyo 7.1 htib(home theater in box) system , they do have some decent ones that sound great for the price, although it wouldnt sound like $1,000 speakers, it could sound as $500-700 speakers.
www.onkyousa.com
and
www.svsound.com are two great places to look. I wouldn't go with aperion audio, but if you can afford axiom, you can try them or onix.
The trouble is you need an amp, so I am not sure what your needs and budget is.