Would you know how to correctly open a CFX20 to clean the pots?

Based on the information you've provided, it sounds like the issue with your Pyramid GA810 guitar amp may be with the preamp section of the circuit. Since you have cleaned the pots and input jacks, it's unlikely that they are causing the problem.
Here are a few things you could try to troubleshoot the preamp section:

1. Check the power supply: Make sure that the preamp section is receiving power. Check the power supply components and ensure that they are functioning correctly.
2. Check the signal path: Using your signal tracer, trace the signal through the preamp circuit and look for any components that may be faulty. Check for cold solder joints, bad components, or broken traces on the circuit board.
3. Check the output section: It's possible that the issue could be with the output section of the circuit, rather than the preamp section. Check the output stage components, including any power transistors, and make sure that they are functioning correctly.
4. Seek professional help: If you are not experienced in working with electronics, it may be best to seek the help of a qualified technician. They will have the necessary expertise and tools to diagnose and repair the issue with your amplifier.

I hope this helps! Let me know if you have any further questions.

Please post some more detail of how each channel is set up and what you have assigned where. My guess is that reading the manual may sort your issues.

What makes you say it works fine if you are getting no output? Have you just got it turned down?

I would spend some time with contact cleaner, apply it to the switch contacts of any 1/4 inch jacks, and all of the connectors to the effects boards- it will entail some disassembly. Check all chassis ground connections you may have one that is a bit loose, broken, or corroded.

The Mackie support downloads for manuals has 2 sections- you needed to look at the top at the "discontinued products" Where these are.

http://www.mackie.com/support/downloads/discontinued.html

http://www.mackie.com/pdf/cfx_om.pdf
http://www.mackie.com/pdf/cfxseries_ss.pdf
http://www.mackie.com/pdf/cfxseries_hu.pdf

http://www.mackie.com/de/pdf/CFXSeries_OM_German.pdf

If headphones have both sides working, it is NOT the main fader. It would most likey be the driver chip for the failed side that has been blown. This is common problem when people fail to power the mixer from the same receptacle or power conditioner that amps and other connected equipment are powered from. Ground bounce damages the circuits. The balanced outputs are driven by NJM4560 op amps which are likely surface mounted 8pin chips... .

Here is service manual:

http://elektrotanya.com/mackie_cfx20_sm.pdf/download.html

Scroll down to "get manual" and click on it to download. Ignore other download links for viewers.

Shafts will SURELY break if you try to straighten them. Replacing these requires labor of removing a lot of knobs and screws... cost of the pots is minimal.

sounds like a blown output transistor

Here is the complete service manual:

http://elektrotanya.com/mackie_cfx20_sm.pdf/download.html

Make sure the PAN knobs are centered...

Also you can do a lot of troubleshooting with headphones in monitor jacks, etc.

Remember to use BALANCED lines out of the mixer which are either XLR or TRS type cables.

If you have the old pots look on the edge of the cans (metal backs) for the code. Take them to a good electronics supplier and they can match them or measure the resistance from each outside pin with all pins free of interconnection. You will also need to know the audio tapers. The supplier should be able to tell from the codes.