Subwoofer Loading - What is to close?

[Sendero]

I've been toying with the idea of rebuilding my current sub box and changing its configuration. Instead of the normal rear seat delete, the sub box occupies the space of the former rear seat bottom. The subs are 10" JL Audio's firing straight up with a Rockford Fosgate Punch 125 amp providing the power. (Its bridged and driving both subs) Each sub has their own 0.8 cu.ft. sealed space to work with. Materials are a mixture of 3/4" MDF and layered Fiberglass.

My complaints are mostly sub response and range. For some reason the box feels like its a half of beat off with the windows up and a full beat off with the windows down. Also, the box doesn't have the gut shaking low range that some of my other boxes have had (with the same subs and amp). I am fairly certain it has to do with my box construction and firing configuration.

What my main question is: Without changing any other components or box size, can I change to a down firing configuration and achieve the proper response and range? How close is too close to the subwoofers (in regards to a solid surface for the sub cone to couple with, example being the distance between the cone and the floor pan? Is there any other components you would change to help my cause?

Majority of my music is played with emphasis on sound instead of teeth rattling bass. Although I do like a little "Salt Shaker" every once in a while.

Any help would be appreciated!