need help bad

[liftedcj7on44s 10+ year member]

Ok here is what i have. 2003 ford taurus. stock motorcraft battery up front. 4ga power 2ga ground running to a 420 amp interstate battery in my trunk 2ga power and ground running to the amp. 60 amp fuse in the power wire running from the 1st batt to the 2nd batt. Amplifier is a infinity reference 1211a. 854x1@4 ohm and 1300x1@ 2 ohm(rms figures) sub is a idmax 12 d4 v3 in a 1.3 cu ft sealed box. HU is a pioneer deh-p4900IB. my problem is that i cant get any more than 27 volts out the speaker terminal before the amp starts to clip(roughly 365 watts or so. sub is wired 2 ohm. i really need help here i have changed everything except for the amp and sub.

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