18hz @ -9db means the frequency is being played 9db lower than the reference level and at -9 down you wouldn't hear it much in relation to the other frequencies, but you will still fell it. Anything below 20hz is said to be subsonic, you don't hear it, you feel it.
The Hsu you are looking at would still have problems with 8hz as well, to get that low with any sort of power and stay clean(no distortion)you now need to be looking at the upper echelon of subs. Even my PB13 - Ultra in 10hz mode is down considerably at 10hz, about 12db IIRC.
This is where all those reviews get Axiom into trouble as they may not play as low but as I said, unless A/B comparing the lower frequency response wasn't missed, at least that has been my experience.
