If your guitar amp is not equipped with a midi interface and you need this feature, you can use an external midi interface / midi switcher. Just connect it into footswitch socket and to your midi controller. Then you can switch your amp by sending midi commands.
Some amps allow the switcher to be powered from the footswitch socket, so in this case no external power supply is needed. This small and smart midi switcher can be mounted on the rear panel of the amp. You can then use any midi cable to connect the switcher into your midi controller using a standard midi cable.

EM Custom

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Review by Boogie Board

MIDI Interface