I think the Rti 12's should be somewhat similar to the Rti A9's. Both have 90db sensitivity, but IMO when I auditioned them (the A9's) they sounded much better and fuller with more power. That's just my subjective opinion, but to me, it seemed like the bass had a lot more oomph with at least 200WPC of good clean power. This is especially important if you want to hit reference levels with those things.
YMMV, but if you go over the polk forums you'll see many owners saying the same thing. However, I would try it first and see how they sound with the 709. You may find that everything is sounds good. In that case there's no reason to spend more money on an external amp. If you have a friend who has an external amp, you can always try an informal A/B comparison. However be careful about level matching otherwise the one with the higher gain = louder will always sound better.