Mojo
Your room sweep is looking better at the higher crossover compared to the first graph you posted but eliminating that 50hz peak is where you should focus your effort. It is likely making your bass sound boomier than normal. You may want to consider an equalizer to tame that peak. I currently use a Velodyne SMS for that problem but have had good experiences with the less expensive Behringer Feedback Destroyer DSP1124P.
As for those pesky nulls, don't even think about Audessey equalization or any equalization for that matter. Room treatments are your only viaable option. Its normal to be concerned about aesthetics or WAF when it comes to treatments but properly done they can actually accent room decor. I'd start with the easy stuff, bass traps in the corners. Keep posting those graphs its neat to see your progress.
Oh yeah, the other suggestion: get a properly placed second 500 and don't look back. That relatively whimpy Sony sub won't cut it for what you are trying to accomplish.
