I have an 09 convertible with Monsoon amp and have been fighting with this the last couple of weeks. When I plugged it all up i had a constant (and annoying) white noise coming from all speakers, even with the volume at zero. Also, my JVC HU volume adjustment goes from 0 to 40. Anything past 6 would risk blowing out the speakers. I found conflicting information online as to whether the stock HU sends line-level or speaker-level output to the Monsoon amp, but obviously I was feeding too strong a signal to the amp.
I called Metra tech support and they said "we usually don't have a problem with the Monsoon systems". Their suggestion was to attach the speaker wires from the Metra harness to RCA plugs (instead of the normal speaker-level outputs of the head unit). Then use the pre-amp/line-out RCA jacks on the JVC. That solved the white-noise issue and the volume range is more normal. Unfortunately that introduced a different kind of static and constant crackle from all speakers that is equally annoying, plus a slight whine that fluctuates with engine RPM.
I called Crutchfield (where I ordered everything for close to $400) and their expert seemed rather stumped and uninterested. He recommended a power line noise filter, which I ordered. Did not make any difference. I'm now debating over 1) bypass the amp by making about 18 splices in the trunk where amp plugs in (this also means using undersized wires that are only intended to feed the amp to drive the speakers), 2) moving the noise filter to the amp power line to see if that helps, or 3) return it all and go back to the stock radio which had none of these issues.
In theory using line-level outputs from the HU should work fine. For all I know this could just be bad/weird wiring in my particular car. Hope that helps.