Greetings from the land of the midnight sun, short of the really massive isolation box trick it all hinges on the direct out circuit design. If it's taken at the speaker you're going to need an audio "non-inductive dummy load" rated at 8 ohms and 100 watts or bigger to replace your speaker to be sure you don't tax your tubes or transformer. Some people say you can run tube amps without a load but I just can't recommend it. I believe some of the commercial power soaks would do the job as well although I'm not for sure on that one.

The words "it's red box" makes me think that it is a speaker tap design.
If it was a preamp tap design like some of the later Fenders you could remove the output tubes (with the power off -of course) and call it good.

keep on pick'n
gb

Get a Hot Plate of appropriate value and set it to "load". It will act as your cabinet.

Marshall SE100 (used on ebay)
Groove Tubes SEII (buy new or used)
These are the best 2 on the market. There are others as well. But these 2 are known as being the better ones.
Here's some more to choose from
THD Hotplate
Marshall PowerBrake

thanks guys

one word motherload....

http://www.motherloadusa.com/motherload-amplifier-load-box-recording-products.asp

The Palmer speaker simulator works really well for this.

(dead link removed)

I have one.

cheers guys!

could you connect to a speaker cab and keep the master volume down and the channel volume up? would this keep the cab silent or am i missing something?