Devious Head Units Causing Chaos? Low sound from CD/AUX. No Factory Amp.

[SuperDuperChevy]

Now I hate to make an introduction onto a forum by asking a question, but I've had it up to my neck dealing with this. First, I've ran a JVC KD-G730 for a few years, never had a single problem. Then one day a couple weeks back I crank my truck back up after stopping for a few minutes, and I notice my sound is quiet and distorted through my four primary speakers, running straight off the HU, but my MTX 12" is doing peachy. I get home, shut the truck off(Still making sound by this point, not as low as now but distorted) go do grown up things, come back to see what I could do and the HU now has no sound coming from the four speakers, but the sub, running from the RCAs on the back, and through a 340w amp, is still as happy as it can be. Well I check all my speakers, check wiring, check power, check ground, check everything I can think of. Nothing's wrong, so I assume after a couple years the internal amp finally went kaput.

SO: Fast forward to yesterday when I finally picked up a new radio, a Dual CD-770 as a temporary replacement. Hook it up, check everything, and it works perfectly. Less bump from the sub though, but perfect. So the next time I crank the truck I notice my front speakers aren't working now, but the back are, so I go about my business, get back home, shut her down to go do more work, and come back later. NOW I HAVE NO SOUND! Except that annoyingly cheerful sub. So I take it back out, bring it in, and hook it to my bench. I'm getting perfect sound out the tuner, but even cranked up to max volume with Loud on all sound output from the CD player is very quiet, I mean incredibly quiet. If this sound output was gas it'd be deadly. HU was working perfectly for the first couple hours I had it in!

I'm nearly at wits end here, I've untied, untaped, tied, and taped so many times today it's all I can think about!

Same thing the JVC did, other than the tuner didn't work on it! I assume internal amp?

The truck is a '90 1500, so there's no factory amp that's causing me issues here. As well, the amp was hooked into the only blue wire(solid) but functioning perfectly fine for a few hours before issues started to occur.

No power antenna, no factory amp, no ground or power issues, no shorts in the wiring, I've tested every individual speaker wire set individually to the same conclusion, and I've nearly thrown a ratchet from frustration.

For those of you who have realized I'm a bit long winded:

HU: Dual CD-770

Vehicle: 1990 1500

Amp: Dual 340w

Sub: MTX 12"

Issue: Incredibly low volume on anything but tuner.

No factory amp/power antenna

Wires are correctly matched, not my first rodeo.

Ground is good and stable, power is correct and constant.

Speakers are good, each tested on CD, then on tuner.

Shoe Size: 10

Frustration Level: OVER 9000!

Mother's maiden name: Marshall

Any advice here? I tried searching all I could, hence the pointing out that there's no factory jibba-jabba(Foo!) to be giving issues here.

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[SuperDuperChevy]

Pretty much narrowed it down to the internal amp in the stereo having went out, wired a regular speaker into my main amp(turned down), and it works perfectly fine. Oh well, time to get a little amp and run all my speakers through it.