Two 210 x 2 amps or One 500 x 2 amp.....

That's a hard question to answer because it is gear dependent and there are valid points on both sides of the issue. Assuming that all amps are made by the same manufacturer, it's pretty common to find that the 500 X 2 amp would have better quality parts resulting in better sound. On the other hand, running vertically bi-amped means that you are not subjecting your mid and upper frequencies to back EMF from the woofers.

There is a tradeoff however because, as I remember, I've heard owners comment on the Aerials responding well to bi-amping. My speakers are tough to control on the bottom end and I found that vertical bi-amping was the best way to accomplish it. If I were you since you already have the Citation, if you haven't already, why don't you hook it up and see how it sounds? If you don't like it, or feel you have enough power, then you'll probably want to upgrade anyways, even if it's going to 2 500 X 2 amps.

The effect varies with different speakers (cross-overs with big impedance swings seem to benefit most from bi-amping). As long as the extra money usually required for cases,parts,wiring, etc. doesn't compromise total system needs. It would seem to me that putting the amps before the cross- overs would make the most sense. The promise of digital cross-overs ( correcting driver to driver anomolies, maintaning time and phase coherence, fine tuning room correction not to mention the cost savings to manufacturers in the amount of time and labor to make speakers with tighter tolerances) may promote individual powered drivers in the near future.

Responding to Unsound, I would think you would want the amplifier after the cross over, that way you each amplifer is responsible for only a range of frequencies instead of them all?

I would think biamping is better because it allows isolation of power supplies, transformers, and all that good stuff. Especially is you are running one amp to each speaker. Just my thoughts...