Dnut999, how did you mount your amplifier? Did you use the typically provided metal screws/brackets directly into the car body?
I ordered the Kenwood SW11 powered subwoofer, the PAC module, etc, but can't find any CX-5 specific info online on what's under the floor covering under the seats. Obviously, screws are more secure, but the Kenwood subwoofer is only about 6.25 lbs and I'm thinking that the Velcro Industrial Strength product line might be adequate, of course depending on how well the Velcro adheres to the floor "carpeting" itself to start. I know that Velcro is frowned upon for the potential to fail or loosen, but given the underseat mounting and the relatively low weight, I suspect there's less risk there even during an accident.
I cut out cardboard to match the SW11 dimensions of 11"W x 2-3/4"H x 7-1/2"D (a bit surprised at how small the footprint is visually using my mockup). It may just fit left to right length-wise next to the vent, which is my preferred mounting direction. Front to back means more of the woofer will stick out and also interfere with the back OEM floor mat. I am on the shorter side, so my driver's seat is set forward, which is another reason why more will be hidden in the left to right placement.
Too bad there isn't enough room to fit it under the back seats (I assume), as that would be an entirely hidden installation even if it means longer cabling/wiring.