voltage tanking on the sxt 15 at full tilt

grab a digital multimeter and measure the voltage at your amp terminals while engine is idling vs at 2k rpm. if you see 14v+ at idle but it plummets to 11.8v when you hit that note, your stock alternator cant output enough current to support the draw at that rpm. throwing lithium in there is just a band-aid that will eventually kill your regulator because it will be constantly trying to charge a depleted bank. i run 3x Fi SP4 18s on an HD8000 and i had to do a 320a Singer alt before anything else made sense. if your voltage stays near 12v with the engine revved up then your alt is tapped out and no battery will fix that. you need the current, not just the reserve capacity.

Before you spend money on cells, inspect your chassis grounds. I learned the hard way with my KX2400 setup that even perfect 1/0 cables cannot compensate for a weak factory ground point under the hood. Take a wire brush to the battery negative to chassis connection and the engine block to chassis point to remove any paint or corrosion. If the metal contact is not clean and shiny, you are choking your current flow before it even reaches the back. Once you verify those connections are solid, use a DC clamp meter to see exactly how many amps you are actually drawing during a sustained bass note compared to the rated output of your stock alternator. If the clamp shows you are exceeding that amp rating while the engine is above 2000 RPM, the alternator is physically incapable of keeping up with your demand regardless of how many batteries you stuff in the trunk. I spent way too much on a secondary battery just to realize my stock wiring was acting like a resistor because of poor ground contact.