So Mark, my new best friend over at Mach5Audio hooked me up with a new IXL18.4 replacing the driver that was ruined by the shippers :/. As stated in an earlier post, Mark's customer service is second to none. He really took excellent care of me.
With the new IXL18.4 now installed, all I have to say is WOW.
The tactile pressure felt deep down into the teens is sensational to say the least. 15 - 45hz sign sweeps deliver the same oscillation feeling that I have grown for in the car audio days providing ample "cloth shaking" sensations throughout your body. I was blown away while watching the the opening scene in Transformers when the helicopter flies by. My pant legs on my Dockers were fluttering.
I threw on a 20hz tone to break in the driver playing it conservatively low. Heading up stairs to grab a drink I realized I had a new problem on my hands. All 3300 sq feet of my 5 level split home was shaking. From windows to door frames to dishes and even the garage door. Laughing, my first thought was "I'm going to need a ton of Dynamat!". My wife now home from work grinned and commented on how it felt like an earthquake.
As they say, there is no replacement for displacement, This thing really moves some serious air. I am so extreamly impressed at how effortlessly this IXL18.4 can drop bottom at high SPL levels without a single sign of stress.
My next steps in the project are to connect my new BFD to it, eqing it with the rest of my system.
I leave you with a few videos I took to augment my feelings about this now finished subwoofer.
I'd like to thank Mark @ Mach5Audio again for your incredibly superb service. If anyone is thinking about building a sub, give Mark a ring. He'll take care of you like nobody else. Thanks again Mark.
Video 1: Excessive rattling all over house caused by Mach5Audio IXL 18.4 LLT. Playing 20hz sinewave. The sound that is heard is not the subwoofer but the rattling throughout the house. The poor quality of this google video doesn't show the mirror vibrating unfortunately. The floor actually flexes.
Video 2: Up close look at driver playing 20hz tone. Again, the Canon camera doesn't pick the low frequency that well, but you can obviously see the nice clean extension this driver has.
Video 3: Rattling door in the same room as LLT, again 20hz tone.
Video 4: More rattling door
You know while at it add three...with four you start to FEEL the powah. 
