Question What kind of cable should I use to connect the G-Major / G-Major 2 / G-System with my Mesa Boogie Amp in order to switch its channels

The current Multi Watt Rectifiers (Reborn series) have been simplified when it comes to the external switching layout.  You can now utilize a Y-cable connection. The same applies to other newer Mesa Boogie amps. Double check with Mesa Boogie to confirm whether this is the case for your specific amp. The following answer applies to these models: (older)Dual and Triple Rectifier, Rectoverb, Lone Star and Lone Star Special, Solo 50, Roadking, Express. According to Mesa Boogie Tech dept, these units require an isolated ground connection for each of the rear panel 1/4" external switching inputs. Since Y-Split cables are sharing the ground, they cannot be used from the G-System or G-Major (which output connection is TRS) because they could cause damage to the amp switching network. You can still use mono cables from the G-Major / G-System to any of these amps. This will obviously divide by 2 the switching possibilities since only 1 relay per output can be used. An other solution is to use a switching device such as AXESS ELECTRONIC CFX4 or MIDI SOLUTION R8 in combination with the G-System. Note that the R8 is midi powered only and requires an adapter available at MIDI SOLUTIONS that can be powered by one of the DC 9V outs of the G-System : The G-System does NOT provide Midi power. Other switching devices: Nobel MS4, VINTECK, RJM Amp Gizmo. The Mark series (Mark 2,3 & 4, Quad, Studio Preamp, formula Preamp, V-Twin rack Preamp) can be used with Y-Split cables.