Don't have the X-13 and only one X-12, which is enough for my house.

Looking at the math you could get two X-12's for less than one X-13 and the two would have almost 30% more radiating surface area though less total amplifier power than the X-13 .

So if the extra couple of Hz extension and the THX certification don't do much for you then I'd be tempted to go for the two X-12s. The one thing that may make it worth it is that the X-13 should have less distortion.

Oh and the X-12 is about 40 lbs lighter smile

That's my 2 cents. As always YMMV.
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975/7075/SMS-1
Aperion Verus Grand Towers & Bookshelves, Verus Forte Center, Infinity Surrounds, Ultra-X12
Oppo BDP-203, XBOX360, Xbox One
LG 65" OLED
RR2150 w/Klipsch SCR-2