Yes, your theory is wrong.
Generally amps put more wattage in to a lower resistance (like the XD which puts 100w (rated) into 2 ohms but 75w into 4 ohms) generally its linear.
However the HD series have switched voltage rails. In which case, as you posted it will put out 500W at anything between 14 ohms. If you take two 2 ohm speakers your options are a 1 ohm load (parallel wiring) or a 4 ohm load (series wiring). Result=500w.
For loads above 4 ohms, it follows the conventional wisdom. So two 4 ohm drivers in series would be 250w.
Now...
If you have decided for the XD, i would get the 2 ohm drivers to get a bit extra power out of it.
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WTB  Used  1x Blizzak LM60 225/45/17
