I don't have the microsub, just the speaker, I am fully aware of the reputation of the microsub, sounds bigger then it is. I am building my own box and it will be close to the microsub spec. Love your feedback, I feel better knowing that it is working well for you.
I am also investigating the possibility of building the sub under the passenger seat. It is possible but requires work. The box has to be bolted to the glide rails so the box moves with the seat when sliding the seat in different positions. The box will also be elevated so it won't take up valueble toe space for rear passengers. This is a big project and I am still thinking it over carefully.
Also the heater duct under the seat has to be removed as this creates a big bump under the seat and gets in the way of the sub. The heater duct only flows a little air when the climate control is set to "floor". I don't have many rear seat passengers.
I have also investigated the idea of the center console enclosure. There is very little room. It could maby be done with a six inch woofer which I believe JL also makes.
I will keep you all up to date. Further comments are appreciated.