I have a single 10" upfiring sub box...
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This is my first experience with underseat type enclosures as I'm used to the much larger forward firing sub boxes.
I'm totally happy with the bass I can get out of this enclosure and it has actually exceeded my expectations. From what I heard, there are a few good points to having a down firing enclosure as opposed an upfiring one like RS mentioned about the sound travel in the cabin, masking higher frequencies that may get through to the sub, and protecting the woofer from potential damage from the seat undercarriage.
Let us know how things go if you decide to get the downfiring enclosure...