Your high level amp sound have at least 2 sets of + - connections. Take each of your speakers + and - wires and hook them into the amp. All youll have to do is either re-run wires to the amp from the speaker or move the existing wire.
I'm not sure if this is for all amp but when I still had my stock deck and I used the high-level input in it sounded like crap. I bought and installed a line output converter and it hit pretty hard. So it was a personal experience suggestion. Has anyone ever compared high level input vs. line output converter?