And just for general information, the head unit will not put out 8V until a certain lvl, and most likely, that lvl will be at an unreasonable volume. All receivers are different, but take for example this Alpine CDA-7893.
All work is that of Jmac, I in no way make claim that it is mine or my work
The rated 4V output on the subwoofer pre-out was not reached until volume 35, which I believe is max volume.