Best thread for this, does not matter much what unit you buy:
https://www.crossfireforum.org/forum...et-stereo.html

Generally speaking, you want to remember that the Crossfire Power Amp was designed to be driven by SPEAKER LEAD OUTPUT from the new unit. Most manuals will tell you to use the "RCA" outputs, but that is WRONG in our case.

After you get it going, try modding the system to four channels here:
https://www.crossfireforum.org/forum...-channels.html

This mod really helps:
https://www.crossfireforum.org/forum...-buck-mod.html