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You will have to keep it way down to keep it from blowing up the surround speakers.
Backflow of that wattage into your other devices could blow amps there also.

It won't hurt your guitar amp to do it unless the ohms rating is wrong, then it can blow the driver ciruit in your amp.

I have done a ton of this kind of experimentation, and currently have a 400 watt Yamaha theater surround system as a sound system for my computer in my office.

For my guitar I had a Peavey Stereo Chorus 400 amp with left and right channel output to separate 3 way Peavey speaker cabinets, each on 50 feet of cable with thier own roll around rack. (stereo amp and 100 feet of separation) and stereo effects pedals.
I sold that because it was just too much, so now I play through my computer and use Creative sound blaster audio output in the computer output to the Yamaha surround system.
I still have and use my Digitech effects pedal RP100A run through the computer also.
I also use headphones a lot of the time now instead of blasting everyone in the neighborhood. :D