Given the diffuse dispersion of sound with the quadpoles, they should be quite forgiving in terms of how high up you mount them relative to the side surrounds. If they're mounted, say, at your listening position height, you'll hear more information from the upper woofer than you would if they were mounted higher up, so losing the lower woofer's output isn't that big a deal.

A 36" stand is certainly adequate, but you can consider different stands as well.