You guys are throwing around 120dB @ 20hz like it's what everyone else has.
I have listened to a movie where the sound averaged around 100dB @ 25hz ~ flat-ish to 20khz @ seating position, and it was pretty damn loud. Not "that's a little loud, can you turn it down?", but I was actually squinting a little. As if changing my ocular disposition would affect the waveforms assaulting my ears. I could have turned the sub up a little more, but it sounded fine where it was.
Now, 120dB @ 20hz? Have you ever heard a hard hitting 20hz? Even a small amount of that is enough to rattle picture frames - hell I hit just under 110dB and the floor felt like it was moving. None of that is normal lol. (Well I think it was around 20hz... its the Tron Legacy scene where buddy enters the Tron world and the big ship picks him up and flies him to the arena. Floor shakes.
Ok here's what I think. Yes, I love 20Hz moving my floor, and people smile when they experience it. However, "LOUD" depends on what your life experiences have ruined you for. If your only sub was a 12" ported from polk audio, then you probably feel like the king of the world when that thing puts out anything more than the TV sound. At that point you are probably barely dipping into the 100dB range at the 40-60Hz peaks. Then you DIY and get a taste of extension and considerable SPL increase. At this point, anything else you can afford at the store will sound like an expensive fartcan with a nice logo.
After hearing everything I have, and being a little bit practical, I would say that for most uses - a sub that reaches 20hz @ 100dB (-3dB groundplane) would give the average spouse a satisfactory sub that usually wouldn't see more than 90dB with the wife/kids around, and when you feel like it, take advantage of the corner loading and put out 100-110dB - which is actually pretty loud.