If you have the real estate, box size increase is generally preferred. I wouldn't go much smaller than 150 liters for ONE of them, and that's pretty small for a 21" driver. If you can double your size, do it.
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Power shouldn't be problem these days, you can get like 14kW for like $700 shipped or whatnot.
I haven't modeled it but a bigger box is probably best in terms of depth and efficiency and cost, just limits placement options and takes up space.
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