Before you go the Def-Tech route, try a few things and give us some info. First, if the room is bigger, and bass is the issue, I don't think you can assume the Denon 700 is supplying plenty of power. But before we let those worms out, try this: Put the Denon in your second room, and test the speakers with a clean short run from the main amps, not the surrounds/zone 2. Mess with Bass and Treble settings to see how much you can gain from the best possible set up and locations. If that set up is not good enough, you know you need to make big changes and can skip all the second zone issues at this point.
If that sounds good, then you have options. Increase the power to zone 2 with a second cheap AVR or buy a stand alone amp (with tone controls). If you notice improvement but still not enough bass, then a separate sub would be the way to to, but you have to work out the zone 2 issues with that. Adding a built-in sub (like Def-Tech) may be a good option in your situation, but be aware, most of what I (just me, don't freak!) have heard and read on the lower-end Def-Techs with built in subs does not cause me to believe you will find your answer there.
Most likely, in my opinion, you would be best served by a sat/sub setup. Smaller speakers higher up to avoid as many reflections, tied to a solid sub to move the mass of air needed to replicate natural sound even at lower levels.
