If it is a 16 Hz tune, you won't need 15 cu ft for the TC 2000, but only around 12 cu ft net will do it.
Nevertheless, I have checked for the RLp 18 in a 15 cu ft net enclosure, and found it will be only 2 db higher than the TC 2000 in a 12 Cu ft (same tune 16 Hz)
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So you're right, for the space, 2 db may not be an issue.
Blaser