Re: Speakers make loud crackling noise at higher volumes
I know you said you used the RCA cable straight to the GTA4 yet still got the same noise. Have you tried doing that since making a common/shared ground?
Personally I think your RCA cables are fine, to answer your question about powering both speakers non-bridged: Yes it should, the noise could be coming from a power wire that's running too close to your RCA's routing into your trunk. I'd look into Shielded RCA's (Kicker KX-Line for example.) If the noise(s) diminish when you eliminate the Y's and run the 1st set of RCA's to your GTA4, I'd say the ground was your issue all along. If not, then it's the amps RCA inputs. If you recently purchased it brand new, replace it. If it was purchased through a 3rd Party (a member on here, craigslist or resale of some sort,) you may want to open it and look at the input/output solder points.
Best of luck and hope I helped.
Team Starr Mountain Nuthugger #4
Bought From: outsideshot23, bigguy2010, SAZCLASSD, centergod79, SPLaudio, JCRF71, Vertex Audio, assassin1840, woofercooker, double 07
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1998 Pontiac Grand Prix GTP Sedan