I would second the Attenuator option. I use a Weber MASS to test amps out at higher volumes when the family is around 
There are other things you can do to reduce the overall gain as well. You can always try a 5751 tube, or even a 12AU7. Of course, that will change your distortion amount as well. You can also change your cathode resistors to a little bigger value to get less gain (this can be a bit tricky as you are changing the tube bias). You could also snap in a resistor in series with the 22n resistor (C6 on the TAN schematic in the wiki). !5K, 33K, 68K would drop the signal going into the power tube.
But to get the same exact-ish sound at lower volumes, then the attenuator is the way to go.
Another option is to get a lower db rated speaker.