passing monsoon amp

hey everyone - i'm stupid when it comes to electronics.... but i have a question i replaced my HU and my speakers in my golf 4. Now the rear speakers are quiet as hell. i was told by a guy who installs audio that's because the rear speakers on the monsson had 2ohms and i put in 4 ohm speakers and theres more resistance or something like that.... what should i do to get the rear speakers to play at a normal sound level... i heard bypass the monsson amp.... HOW WOULD THAT BE DONE?
thanx