Am I reading this right, you wanna try 1.5 cu for a XL 15? I can tell you this must. I dont see an issue running 2 xl's on a 5k. Even in a prefect world, if the subs seen the whole 5k of clean power, you dont have enough to destroy the XL's from overpowering. Now if your clipping them to hell and back then you will have a problem. I have found where my XL 15's come apart at with too much power. But with a few changes there going strong and doing way more damage then before.