Of course, you can't hear it, but if it makes your face tickle when you crank it, it's gotta be good. It'll hit 8 hz without serious roll-off, so if you have the cash, go for it. Check the data sheet http://ww2.bagend.com/bagend/d18e-i.pdf They also make the Infrasub 12" which will also take you down to 8 hz, but it rolls off steeply after 10 hz. SVS CS+ 16-46 with port plugs will hit about 8~10hz, but I might be wrong about that. I am still waiting for my SVS. 
Bag End Infrasub 18". It hit's 8 hz without problems
Unless, of course, you expect to get any usable volume from it. Over at Link Removed, it indeed measured flat (more or less) down to about 10 Hz in the room response test. The kicker? They measured a maximum of a measly 103.5 dB at 31.5 Hz using nearfield testing. So, unless your key seat is staring the woofer in the face, you're looking at a much lower peak than even 103.5 dB. Even on its web page it states that "the maximum SPL is not a reliable way to judge the impact of a subwoofer".
That pretty much rules it out of any use for me, excepting elevator music that reaches down to 8 Hz
Yeah, you notice, I didn't say I was waiting on my Bag End, I said I was waiting on my SVS. It's kind of scary to think that 800 watts driving a pair of 18" subs can only hit 103.5 dbs at 31 hz (it should do that at 15 hz), but heah, if you are into that "mine can do it and your's can't" bass elitist thing, then there's the sub for you. I thought it was the Infra-12" that didn't have much in the way of output, but I might be wrong. Never considered the Bag End for long because it was cost prohibitive, and I read some of the same things you have about them.
