JohnH

As long as you start with volumes low, and move up carefully, theres no risk to the gear. You may have a level mismatch, with a very large signal from the Mg line out doing too much overdrive of the second amp.

The line out from the Mg is probably "cab simulated" which means it is filtered to have the response of a guitar speaker (more bass, mid dip and high treble cut), so it can go directly to a full range PA or recording set up. Taking that signal and feeding back through another amp and cab will give it a double dose of guitar cab sound - it may be a bit wooly sounding. Or it may be great and awesomely heavy.

If you want a more honest setup, ie amplify the tone of the Mg through the valve amp, without asing more preamp or cab simulation, a better way, if you have these connections, would be a cord from Mg effects send to second amp effects return. These are more likley to be using similar signal levels, and do not involve extra filtering.

John