Stereo help (pretty basic)

OK this is in my 67 chevelle. I am running a HU with front and rear RCA hookups. The front RCA's the front channel (two 6.5s) of my 4 channel amp. The rear HU channel is split to two sets of RCAs. 1 set goes to the 4 channel rear for two 6x9s. The other set goes to a mono Boss amp for a 12" sub. Both amps have dedicated ground and battery wires of car audio quality (fused properly). grounds to body and + to Battery in the trunk.

I hooked it all up yesterday and got nothing out of my sub/Boss amp. The fosgate 4 channel was working fine with good sound to the rear channel 6x9s and front channel sound to a 6.5 test mule driver. Although once I started the car I got very low volume out of the 6x9s off the 4 channel.

I called Boss tech support and stated I get 9.4 volts to my remote wire on the Boss amp. He said I need 12 volts to that rem wire or I won't get the amp to turn on, although I have the on LED working, I figure the fosgate is getting the same 9.4 because the wire is split with a butt connector right after the HU to two REM wires.

He suggested I get a 30amp relay to power the HU and both amp REM wires.

I also got alot of speaker whine and alternator noise which he said was from my RCAs being close to the power supply, easily fixable.

So since there is lots of experience on here with stereos will a relay solve my problem? I thought I read in the directions the rem wire only needs 3VDC, but maybe I read that wrong. My tester said 9 for sure, with battery connection at the Boss Amp at 11.94, since I had been testing with key to accessory and wore the battery down a bit.

I have wired my boat stereo the same way with 4 tower speakers off one amp and my sub off another and it has always worked great. I don't have a relay for the REM wire I just split it as I attempted to in the chevelle. Then again boat is an 06 and chevelle is 67 so maybe my alternator in chevelle needs an upgrade, not sure what the Black Scorpion 350 alternator is rated at in the Sanger.

The chevelle stereo is not trying to be anything crazy, just some sound for cruising, hence the $50 boss amp and $60 kicker sub.

Thanks in advance for your help