High pitch sound coming from speaker when turned on (sounds like feedback but not very loud)

The short answer is yes. You didn't tell us what make/model/speakers, but if it was really high volume you may hsve blown a speaker. Of course, if you let it get out of hand you can damage your ears.

If this happens only when the guitar is plugged in, most likely there are two possibilities. The first, you have some issues with the electronics in the guitar OR are using an acoustic guitar witch has an extremely sensitive pickup that provides nothing but feedback. If this is happening with the guitar unplugged, there is a mountain of possibilities.

you are sending the signal back on itself, hit the usb to main switch to off.

Make sure the guitar CANNOT "HEAR" the amp you are using. Local regeneration (feedback below the level to sustain full feedback) will give a boom sound like in a cave. If you get near enough to the amp for feedback to build up, you can actually see the string vibrating and building up. The loudness has much to do with the amp you connect this too and the settings

Proper level setting of the mixer is important. HOWEVER if the clipping is occuring at the speakers the only possible problem MIGHT be that you have a supersonic feedback that is saturating the speakers above your hearing range, Be sure you don't boost the highs too much with the EQ as this can cause the feedback that you can't hear. Also make sure that you power the speakers from the SAME receptacle as the mixer, even if it means running an extension cord to bring power to the mixer. This is to avoid a low frequency hum and common mode distortion/damage. A low frequency hum could cause the clipping.
It would be a good idea to get a sound meter to check the sound level. You should be able to reach 85 Db from this system without clipping. If you need more than that, you MAY need more speakers if the band instruments are too loud. Also if the band has amps that get into the vocal microphones that adds to the clipping level... make sure the mics don't "hear" the band instruments. Make sure your speakers are toward the audience from the mics to avoid the supersonic feedback problem. If the vocalists can't hear themselves with that configuration you need to set up seperate stage monitors.

There are some pretty good videos on YouTube about proper mixer level setup.

The speakers are 8 ohm so you can probably get near the 400 Watt per side when connected to this mixer. You don't need bridge mode to get this and I would recommend not using it anyway. Whatever you do make sure the speakers don't get disconnected when running at high volumes as you will arc out the amp in the mixer... I know this as I fix them... I have two PMP5000's that I repaired. Lots of work to repair...

The feedback is NOT the fault of the mixer or speakers but instead of something picking up sound from the speakers and sending back to the mixer.

The mic must be well in back of the speakers (say at least 10 feet) to avoid this or you need to use anti-feedback hardware to avoid the problem or highly directional close talking mics. Reverberation off walls can also get back to mics and cause problem. I was able to use a wireless Peavey mic which seemed to disrupt the phase enough to never get into an oscillation even when in front of the speakers.

If the organ has a reverb spring "tank" in it, that is likely where the spring sound comes from.

The initial noise is likely due to the initial unbalance of the power audio amp.

Lesser organs don't disconnect the speakers till the amps attain balance so the inrush pulses the speakers..

What you're hearing is FEEDBACK caused by the nearness of the guitar to the amplifier speaker. Always mute it when not in use, or don't move it close to the guitar amplifier speaker when unmuted.

Usually, even though you hear a high-pitched screech, the problem is actually caused by low frequencies, sometimes below the human hearing threshhold (i.e., subsonic) because they are more omnidirectional and thus more easily easily can loop between input (the guitar pickup, especially on a hollow-body guitar) and output (the speaker). The screech frequency is determined by the distance between input and output, determining how quickly the output reaches the input (then loops around through the amplifier). Therefore, reducing the BASS a bit will often alleviate some of the problem.

This is basically the same type of feedback heard when a microphone is turned up too loudly or the mic is pointed at the PA speaker. Avoid those actions and your feedback will be minimized.

Only time and experience will allow you to automatically take actions to avoid feedback. I worked in the San Francisco Bay Area as a manager and sound man for rock bands and several nightclubs for 22 years. My college major was Physics (which includes acoustics).

Keep on rockin'!

Sounds like your master volume is turned up too much. The channel volume will make the squealing go away(caused by high gain and humbuckers). The best thing to do is dial in your favorite presets and change the channel volume to the correct level with the master volume where you usually keep it. I use my Spider Valve at master volume at about noon and the channel volume between nine o'clock and noon. Let me know if this does not solve the problem......

Change the audio amplifier ic are fried