Here's what happens in standby. There are several taps off the power transformer for various tasks in various voltages. IIRC the voltage for the filament in the tubes (the thing that produces the heat that allows the reaction within the tubes to occur) is 6.3V in many cases. When in standby, the only thing going on is those filaments are on, and nothing else. So, technically, you would be fine.
But here's what I suggest. There is no way that the GK700RB will sound so horrible that you just can't stand it for one more song. (They are actually really nice amps.) So what I would do is just have the SVT set up and ready to go. If you suddenly decide you can't take the GK any more, just reach back and turn on standby and play one more song. It really only takes 10 to 20 seconds to warm them up good. That way you won't risk accidentally turning the thing on all the way and doing some real damage.
Nobody in the audience will even recognize the difference if/when you switch amps. So stop sweating over it.