Multiple subwoofers work wonders

Good for you! I think of multiple subs in a couple ways:

Like room vents for heat — the more you have, the more even the dispersion. Just the temp you want without any fanfare from any particular vent.

In terms of cancellation. There were some things I simply could not achieve without the subs *interacting with one another,* using phase to help mollify difficult non-room generated bumps.
 

Adding a distributed bass array was the single biggest improvement to my system.

And, looking at your system page, that is a very impressive and flat frequency response you have achieved. Totally jealous.

Thanks. It's taken me a long time and is still a work in progress. Cannot stop tweaking.
One of the more interesting phenomena I've noted is that while I have been able to achieve an even flatter frequency response, the room modes were far worse. It's kind of a balancing act. 

@tcutter I see a lot of visual symmetry in the room which is quite pleasing to the eye. Did you make some aural compromises to achieve this? Or does it just happen to sound better AND be visually symmetrical?

Little physics, little aesthetics, lot of luck. Anal retentiveness probably helps. 

+1 distributed bass array (James Romeyn's version as well)

not to derail the topic, but I gotta ask: how do you like your Vinnie Rossi pre w/ your XA60.8s???

Just lovely. Tube sound was always offset by fear of reliability (had a bad experience when I was younger), but 300Bs in a preamp are pretty unstressed. It's a dual mono class A design as are the amps. In typical Pass fashion, they have a sound that has been described as tube-like, so they all play well together. The amps are direct coupled to the Wilson Benesch midrange and the music and the imaging is an immersion experience.  Very nice combination.
Midrange rules!  But bass is also nice. 

@tcutter thank you!