Jack,
Yes, Greg is a great guy. I'll get to hear his setup next month. The DIY Atlanta event is about 3 miles from his house. Think there may be a few horns in that place?
Bill, if your wall is 8'x16' and you can use about 90% of it's area, you only need 1mm from the ESL to generate 120dB @ 20Hz.
FYI, that's instant ear damage. And probably a concussion when your roof caves in on you.
120dB in the voice frequencies would quickly damage your hearing (not instantly, but quickly). But at 20hz I think it would take a while. If it didn't, Tom Vodhanel, Bob Sorel and James W Johnson would have gone deaf a long time ago.
Also if 120dB caused instant hearing loss then anyone who attends a full orchestra could be in trouble, as peaks of 130dB in the front rows aren't unheard of.
You have to be careful with your hearing, and I am, but short bursts of bass in excess of a 120dB aren't a problem.