Aftermarket HU install with factory amp options

Hey everyone, First, let me apologize for not being too well versed in car audio and installation. I'll try to be as simple and descriptive as I can. I recently purchased a BOSS-560BRGB HU from Crutchfield and installed it in my 2005 Pontiac Vibe to expand my options for input devices. Unfortunately for me, I did not realize my vehicle had the "monsoon" sound system meaning it has a factory amp. I noticed this pretty fast when I nearly blew my speakers starting up the system for the first time at volume 10/40. Currently volume 0 is a good listening volume, so i definitely knew something was wrong. I did some research and found I may be in need of an LOC unit to run between the aftermarket HU and the vehicle harness so I can adjust the gain down from the factory amp. However, what I don't fully understand is my options in doing this. From what I can tell, my vehicle does not use RCA outputs from the amp, instead all the connections are simply running through the factory harness. From my understanding most all LOC units will convert the RCA connection from the amp to a wire connection to attach to the HU harness. However, it seems I either need the opposite of this, or simply a way to adjust the gain without converting the line to RCA at all. I went through Crutchfield and they recommend the "Scosche SLC4 Line Output Converter" as it can do just this, but the reviews for that device are not very affirming. I am wondering what my best, and cheapest, course of action may be here. Thank you all!