There are actually two ways to switch between channel A and channel B using the HD500 via the Link6 interface. The first is to assign a footwitch to enable / disable the amp model block in your HD500 signal chain. Hitting the footswitch will turn off the HD500 amp model and switch the DT to channel B. When you re-enable the HD500 amp model, the DT will switch back to channel A.
The second way is to set one of the footswitches to send a MIDI CC message 66 with any value 0-127 to the DT amp. This will toggle between channel A and channel B. Alternatively you can send a MIDI CC message 19 with a value between 0 and 63 to select channel A, or a value of 64-127 to select channel B.
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