First off, pull the chassis and find that roaming nut. *That can short out things QUICKLY. *Get it the hellout of there and locate where it belongs (it didn't just 'beam' there from the Enterprise). *Check for "brown spots" on the board inside. *This will show you if the wandering nut caused any arc's or shorts during it's adventures inside.
Next, check ALL the tubes...preamp and power sections. *Make sure they all glow. *This is a quick check and not a true "testing" of the tubes, but....sometimes folks miss the simplest things (I know I have! "oh- the parking brake is on?")
. *Your amp tech should have equipment for testing tubes. *If he doesn't, move on.
American or Japanese or Turkish or Austrailian, a good tube amp
tech shouldn't have difficulty diagnosing the trouble. *See if you can find a good tech to help you out.
In the future, I'd reccommend getting your tubes elsewhere. *www.TubeStore.com
ships internationally. *Nice folks there with a good selection of quality product.
In the meantime, if you can't repair the amp in time, consider renting one. *I know in the USA there are many audio eqiupment rental houses that will rent you an amp for a night. *Check at the local music store
too. *It may cost a few bucks, but at least you have an amp for the gig.