Little back story...made a custom box for my 12" Focal K2 sub for my F150. Focal wants to see between 0.89 to 1.23 cu.ft. for a sealed box. I am right at 1.3 so am happy with that. Providing the power is a Hertz HCP 1DK (1240 W @ 2 ohms, 740 W @ 4 ohms). RMS on the woofer is 800, 1600 max.
I have always wired my subs for the lowest impedance that the amplifier is rated for. Of course, I first wired it in parallel. Even with the gain barely at 1/3rd the sub was just too much. Don't get me wrong...the low lows sounded amazing but I was getting just too much muddy/boomy bass around the 45hz range. Even messing around with my Audison AP Forza 8.9 Bit I couldn't get rid of it. OK...lets try something different. Swapped the wiring over to series and now it's damn near perfect! The lows still shake you to the bone and the drums in rock music you can feel in your chest.
Is this a common practice? Haven't really seen much at all on wiring a sub to a higher ohm load. One other thing I was considering is decreasing the internal volume of the box a little at a time and see how that affects the sound before I switched to an 8 ohm load.