I don't have the schematics for the multi-watt version of the dual and triple Recto, but I've fixed two Triples very recently that both had switching issues.
If there's ANY similarity at all between the switching systems I could probably have that sorted out in about 15 minutes. I'd look at the input and output signals going to the IC that controls the switching, located near the back left corner of the main board as seen with the chassis on the bench, electronic side up, knobs forward.
I'd start by unplugging the ribbon connector from the back board where the loop jacks are. That removes all possible input signals that control the loop.
Then if the loop was still on then it'd be most likely to be that switching control IC.
As I recall, the multiwatt units used relays instead of FETs for switching. You may have a stuck loop relay. Wherever that is. See if there's voltage on the relay coil. If it's at coil voltage then something is commanding it. Probably that IC.
I could be more helpful with the schematic to reference, but Mesa has not yet released the multi-watt schematics to anybody. (That I am aware of.)