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First of all confirm if the speaker impedance is not lower then the recommended Ohms at the back of the Mixer speaker socket.You can use an 8 Ohms speaker on a 4 Ohms amplifier but not vise versa . Next see if the fan on the mixer works. If not the Amp is getting very hot and this will lead to unwanted distortion. You can confirm if its a heat related issue by playing the mixer at low volumes for more than an hour and see if it distorts.
Since you have tried all external possibilities, this will require troubleshooting. If you are not adept in electronics, you best take this to a shop familiar with this model.
If I were troublleshooting this, I would suspect the power amp transistors or connections to them might be bad, but the testing would likely use an oscilloscope to trace the signals in an expedient manner..
One schematic for Crate I found uses darlington transistors TIP142 and TIP147.
initially you want to think bad solder joint, but it think its a bad mosfet or rectifier or diode. it would be hard for an untrained person to identify your problem let alone an experienced tech. it may be time to move on to another amp. in my shop it would be at least 65 bucks for the repair. so where did the "heat"/energy go from that point and what parts were affected. there may be a domino affect. burn one part, not usually. yostamplifier.com
You can reach the limiting IRRESPECTIVE of where the volume control is set when you have a guitar or other source that has HIGH output.
Guitars that have battery in them with internal amplifiers are an example of a high output guitar. Plain inductive pickups may require the volume control to be set at say 70% to reach limiting.
Please NOTE that the volume control is NOT linear, but is an "audio taper".
Limiting means you are probably driving it too hard. Many learn the hard way when speakers and thee power amp fail.
When the cone is driven too far beyond the pole pieces of the magnet, then the IMPEDANCE of the speaker becomes relatively low and can damage the power amp. The limiter tries to prevent this by sensing the current and or voltage and will back off the gain internally. This causes distortion when it happens.