yes i see that picture in the manual, i think you understood that photo wrong. the little ohm number next to the sub is telling you that you should use a sub that is 2 ohms or greater. when running 2 subs they need to be 4 ohms or greater. you have been running at 4 ohms. as for what you did to make it sound terrible i think you had it shorted but the amp never kicked into protection.
those terminals are the same channel. its a mono amp but it allows for two connection per polarity. so the ++ is internally connected to each other. and the -- is connect to one another.
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope