What only one 15" for generating bass..............
No wonder you needed a lot more drive from the amp.
With bass units you are generally better employing multiples as if they need to excurt enough to easily be visible doing so then they will not display the same speed and lack of hangover giving rise to the lack of dynamics. Also whilst I am at it woofers facing the floor never worked for me as they slow down the wavefront to an unaceptable degree for me!
Just to complete this view you really need a minimum of 4 x 15" drivers to convey an effortless realism in the bass and keep transients clean, any less and you cannot move sufficient air to create a realistic wavefront unless you employ horn loaded drivers.
The Kevlar bass unit you employ may not be the fastest in the focal range either. I learnt from my Meridain actives that 4 x 5 " kef units actively driven per side can provide fast bass down to resonance with music and that larger bass units (such as 18" drivers) tend to sound slow in comparison. I have 16 such units and when brought in beneath JBL kick cabs slows down the sound despite having the best BL of any driver of that size!
Thinking about this further you are deploying this arrangement for HT use so it's all the grumbly stuff in movies as opposed to music so it will be less critical audio wise. Have you tried simply turning the cabinet on its side to have the driver facing forward?
To leave this on a positve note the cabinet with sand damping all sounds very good.
MDF is heavy, did you brace this internally too?
With bass, energy is wasted flapping walls but you know all this due to your other sterling designs.
What velocity does the port reach at the box tuning and what is the final Q?
Any lossy damping internally will also slow down the sound.
Do you have suspended wooden floors or concrete? Tuning to such a low frequency may also impact upon the speed of the final outcome, sometimes an IB can be prefferential in speed terms for such LF duty. All IMV (SNA) but based upon self builds and experiance. Final point, Class D amps, the new boys on the bock have very high damping factors and grip on the dirver and can enhance the speed / transient delivary of the driver making them quite suitable for low bass drivers even when ported.
V