New Headunit, Amp Question
The rewiring he is talking about is done at the head unit - I have a friend who worked at CC as well, and it's fairly common since a lot of systems with external amps are not looking for speaker level inputs (Monsoon is): in short, you run RCA ends from the pre-outs on the back of the HU, and either splice or solder the appropriate part of the signal on the other end into corresponding speaker wires.
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I think tasilver hit it right on. The factory Monsoon system uses an ordinary head unit that sends the speaker level output through an external amp to the speakers. This amp is what we used to call a "booster" because it adds amplification to the already amplified speaker signal from the head unit. If you have a head unit that puts out significantly more power than stock then the amp will multiply that and make the volume go up geometrically.
To use round numbers (that aren't based in reality) - suppose each click of the factory unit adds 2 watts to the output and the amp doubles whatever input it gets. Going from 5 to 6 on the volume control would mean going from 10 to 12 watts from the head unit and 20 to 24 watts at the amp output. Now if your new head unit is 50% more powerful it will change 3 watts for each click of the volume control. So, going from 5 to 6 would be going from 15 to 18 watts from the head unit (already louder) and from 30 to 36 watts at the amp output - significantly more. Again, these number are for illustration only.
This is one of the very few situations where bypassing the factory amp can be helpful. Since your new head unit seems to be powerful enough, you could bypass the factory amp to power the speakers directly from the HU. Or, if you like bass, you could bypass the door and hatch speakers but leave the sail panel subs running off the amp. There was a thread a while back with details about bypassing the factory amp: https://ls1tech.com/forums/wiring-stereo-electronics/547488-easy-monsoon-amplifier-bypass-wiring.html
On the other hand, if you don't mind that the volume goes up so quickly and you were just wondering why, then there's no reason you can't leave it the way it is. It isn't necessary to bypass the amp just to run an aftermarket HU - in fact, I don't recommend it except in special cases like volume control problems or if you replace the factory amp with an aftermarket one.





