I would rather have to much and then just cut it back a little if needed. That is how I designed my systems. That usually=cleaner output, lower or no distortion when things get pushed a little. You should also get a more even room response adding an additional sub to the existing set up. Your room is not small. I say start building. The only difficult part is getting them dialed in together since they are different subs. Some use different subs together just fine.
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