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 1-4 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.
If you have decided for the XD, i would get the 2 ohm drivers to get a bit extra power out of it.
FS:Sub Amp - works w/ Technic harness! MRD-M300 - cheap