You need yourself an attenuator. It allows your amp to run at higher volumes (pushing the tubes) but attenuates the volume to an acceptable level.
There are many attenuators available. If you have the money I would highly recommend the Dr. Z Air Brake. It's built like a tank, has fine controls, doesn't suck the treble out of your tone like many tend to do, it will hold it's value well, and will be a box you will hold onto for years.
http://www.drzamps.com/airbrake.html
One on Ebay right now 17hrs left.
http://cgi.ebay.com/Z-Airbrake-Dr-Z-...3%3A1|294%3A50