Thanks a lot. According to the TC knowledgebase there are some compatability issues:
"This answer is valid for all these models: Dual and Triple Rectifier, Rectoverb, Lone Star and Lone Star Special, Solo 50, Roadking.
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 GMajor / G-System to any of this amp. 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.
The Mark series (Mark 2,3 & 4, Quad, Studio Preamp, formula Preamp, V-Twin rack Preamp) can be used with Y-Split cables".
:?
I think I'll check out the thread on the Gear Page for more info.