Feedback squeal means the mic and the speaker(s) are connected via a direct wire connect or via an acoustic loop where the speaker sound reaches the mic which sends it to the speaker which sends it to the mic...etc. Completely muffle the mic, see if that helps, or remove it temporarily and see if it squeals. If it squeals without the mic in the system, you've got a crosswire between speakers and mic lines, probably an upper or lower cable to replace. If it does not make that sound with the mic out of the system, it's acoustical coupling, and you can tighten up the speakers so they don't reach the mic. This is possible more so when there's no 'head' in the helmet to separate the mic and speakers acoustically.
I would also agree that speaker placement is 99% of how a headset sounds, no matter who made it. NO matter the ultimate performance a speaker can deliver, you can lose half of that if it's half an inch away or off-center of the ear.
Since your GPS sounded ok to you, and I assume intercom, AM/FM and CB were not sounding good, there are a couple things that you might check. At times, I've seen the mute button 'stick' even tho it is in the 'unmuted' position, the internals can still be set for 'muted'. That would be same for AUX however, since mute button mutes everything. Probably not your issue here.
Also, make sure the Ambient settings on the bike are turned all the way off. That sounds nice, but reduces overall performance. If you maxed out all the volumes for Intercom and CB and they didn't sound good while AUX input with your GPS DID sound good, it's got to be something other than the headset itself. The headset has no way to distinguish from AUX, CB, AM/FM, Nav, etc. It's all one stream coming out of the factory headset jack.
If you have one dead speaker (or a wire not allowing one to play) or a marginal one not performing, your perception of volume will be far less than with two full performance speakers working together. Ears hear exponentially, so a small change is magnified CONSIDERABLY.
Also, if one speaker is out of phase with the other, your ears won't perceive the low frequency sounds very well at all. Normally that's a big issue in powerful home or professional setups, but I can definitely tell the difference when I purposely reverse (by force) one of the speaker connectors, which normally won't plug in the wrong way. The brain likes both to be in phase, and bass content is almost always coded equally in the two channels.
And now, for the weird class of possible problems.....I was sitting in a parking lot one day on my 1800, and pressed the AM/FM button, and the system went nuts, I was not able to control anything with the audio system at all, NOTHING. It was giving weird displays on the little monitor and such. In the end, it turned out to be a grain of sand got in between the AM/FM button and the button didn't come back from being pressed as the grain held it in a connected mode.... I got my handy dandy little stainless steel toothpick, flicked the grain of sand out of the way, and the bike came back to life. Might want to check that all the buttons are actually working...
Just a couple things to think about.