Power off the amp, and disconnect the mains supply.
Remove the back panel of the amplifier to access the resistor closest to the speaker out jack, which sends a powered signal to an external speaker cabinet. Speaker out is used for connecting amp heads to cabs.
Remove the existing resistor in the speaker out jack. Replace that resistor for one of a significantly higher value. For example, the musical electronics website "Single Coil" suggests replacing a 100-ohm resistor with a 470-ohm resistor. Experiment with two or three resistor values to achieve a good line level.
Keep the speaker wire connected to the speaker, otherwise the line out circuit is not complete.
Wire a black wire from the jack lug to the chassis to ground the jack to the chassis. Consult the manual for further information. This process attenuates the signal from a powered one to a line-level signal that is suitable for routing to a mixer. Treat the speaker out jack as a line out.
Set the multimeter to the "Resistance" setting, and put the probe on the wire that runs from the inside of the modified jack to the speaker. Turn on the amplifier. The multimeter should display a reading that is significantly higher than the value of the old resistor. The resistance should be the same value as the new resistor. Turn off the amplifier. The increase in resistance sufficiently reduces the level of the signal to send it to a powered unit, such as a mixer or audio interface.
Replace the rear panel of the amplifier. Make sure the jack is secure.
Connect a ¼-inch cable between the modified jack and a mixer. Turn on the amplifier and play. The new line sends an attenuated, line-level signal to the mixer. No sound comes out the amplifier even though the speaker remains connected.