Myth #1. Subwoofer size has ABSOLUTLY no effect on "speed." A 15" sub will play the same as a 10" or 12" even 18". An improperly designed enclosure is to blame for double bass drums muddling together. Having said that, if you have space restrictions, a 12" would probably be a safer bet. Have you measured what fits? Or at least what space you're willing to give up? On a properly designed transducer this is correct. But take a motor designed for a 12 and throw a 18 inch moving mass coupled with it's air load and you have transient issues. There is a reason why the motor on a JL w7 8 is smaller than the motor on a JL w7 13. The Ethos motor was designed for a 15 and the enclosures I recommend ensure proper response. This is why every single customer that has bought an Ethos regardless the size says its one of the best sounding subwoofer they have ever owned.