Great, detailed feedback. Just asking the obvious...are you totally happy with how solidly each Ultra 5 sits under the Alexia? With 4 feet and even a slight warble in the floor surface, or one screw turn higher/lower than the others and with one leg having slightly less weight than the others (which can happen with 4 legs), and one can definitely end up with a soft diffuse feel...because as you say the vibration control is weakened by the wobbliness...and yet the four feet will still provide some insulation from the floor reverb.
The reason I ask is because the effect you describe is not consistent with the comments from a lot of people who have them under their Wilsons (incl Bruce who has Alexias as well). BTW, I am NOT saying you're wrong (AT ALL!)...I am simply asking to ensure you get what you want out of this experiment. Nothing is perfect, and maybe Ultra 5s just are a little off for your system. I am just respectfully double-checking, that's all.
(I have to admit, I 'cheated' since the Distributor came to set mine up which made things a lot easier.)
Also, FWIW, I just tried a 4th Ultra 5 under my Zanden transport, but decided against it when I realized the 4th had a lot less weight on it (it slid back and forth too easily), and I did not feel like shifting all 4 feet around and around until the weight distribution was even across all the feet.