The X15's only dig a tiny bit lower and you generally loose speed & precision with larger drivers which can hurt musical performance. I chose 2 x X12 over X15 as 12" drivers offer a better balance for music and movies imo. I just got my second yesterday so, I can't give impressions yet but, even one X12 could completely rumble my medium/small room(17.5x11.5x8') with ease during movies. I suspect 2 will be nuts. If I ever need more or lower extension, I will add 2 more or 1 more with a very larger driver(15+") for the extreme low extension.
Just to chime in on SVS...
When trying to decide between 2 x X12 or 2 X PB-2000 Pro I was told by two different people that auditioned both side by side that the Defiance was more precise and better sounding overall but, didn't dig quite as deep. I chose the Paras as 2.2 music is a bit more important to me then movies, it matches my Para system including ART Surrounds, I got a better price, and they are made in Canada like most of the rest of my system.
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