Hey everyone! I hope this is the right forum for this...
I want to try the following setup with my band, as well as a solo act, but before I do, I wanted to make sure it won't potentially damage my tube amp. Suggestions and comments are welcome! Set up is as follows:

Fender Strat ---> Pedal Board (Tuner, Wah, OD, Distortion, Line 6 DL4) --> Mixer (input 1)
Also, Mic --> Mixer (input 2)

Now, this is a firewire mixer connected to my Macbook Pro running Mainstage. I am looking to upgrade to Mainstage 2, because of how you can sync different instances of Loopback, Mainstage's looping plugin, to, Ultrabeat, a drum sequencer (I have tried to play a loop from the DL4 to a drum pattern, but as they are not synced, it is only a matter of seconds before the loops completely get out of sync with the drums).
I am going to be using midi controllers to alter the sound of both guitars and vocals ( i.e add FX's from Mainstage's arsenal), as well as triggering drumbeats.
So far so good.
Now, for the output, I pan eveything that isn't guitar (vocals, drums, etc) hard right, and the guitar signal post Mainstage gets panned hard left. The right output goes to the FOH mixer, the left output goes to my Fender Twin Amp. Since the signal coming out of my mixer is not instrument level, but, I imagine, line level, would this damage the Twin? I've taken the following from the Amp manual, I don't know if it has anything to do with this:
Input 1: High impedence, high sensitivity plug-in connection for instruments.
INput 2: A plug-in connection for instruments. This input exhibits less input impedence and sensitivity than INPUT 1, making it useful for guitar that have active pickups. This input will provide a darker tone if used with guitars that have passive pickups.

Does this mean I better run the mixer's guitar output into the Twin's Input 2? Should I consider maybe running it through the FX return rather than the input?

The reason why this setup is useful to me is because I want to be able to use the twin in bigger venues and band situations, and leave the amp home and use Mainstage 2's amp modelling plug-in for smaller venues/solo gigs. (Any Fender Twin user will probably agree that using it in small gigs is kind of overkill, and when you don't have a car, carrying the Twin to EVERY show you play is a killer, given the weight of the amp).

Any help/recommendations/comments are welcome, I want to hear your views!

That seems.. overly complex.

But if you're running a line level signal you're probably best running that into the return on the twin's FX loop. But that removes all point of having the Twin in the first place and you might as well just get a power amp and a cab and do it that way.
My band
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My idea was that when I found myself in band situations I could get nice 'real amp' tube sound that you can only get when turning the Fender way past the levels necessary in a pub gig, especially a solo gig. For smaller gigs I wanted to use the amp simulation plug ins available in Mainstage.
Why would plugging it in the FX loop return remove the point of having the twin? I wouldn't use the amp simulation plug ins when using the twin, as I like that tone over the simulation.
Thanks for your reply!
Don't worry about "instrument level" vs "line level." You can control the output level of the left channel. It will go all the way to zero, right? So, just set it low enough so that it's not overdriving the input of the Twin and you should be fine. Ideally, you want to set the level so that it's about the same volume as plugging straight in to the amp with your guitar.

I can see what you're doing there, but if it were me, I'd keep the guitar signal out of the computer. The more stuff your signal has to go thru, the less pure it becomes. That applies to pedals, gain stages in an amp, and certainly A/D-D/A converters and sound cards.
Bruce Clement
BC Audio Hand Crafted Performance