Wattage of a tube amp can be deceiving. Tube amps are rated at "clean" power, yet most people like to drive them to the point of breakup to get better responsiveness (is that a word?) out of them.
In order to double the volume of a tube amp, you need to increase the rated watts by ten times.
So, if Warren buys his half watt amp, which is .5 watt, my Valve Jr, which puts out 5 watts, will be twice as loud, and someone else's 50 watt Marshall, or Traynor Blue, or what have you, will be twice as loud as my Valve Jr....
My point being this..... most people buy a 20 or 30 watt tube amp thinking it will be quiet in the bedroom at night, and they can crank it w/o disturbing anyone...... if you don't have a master volume, gain knob, attenuator, etc.... it's going to be LOUD if it's even 5 watts, you're going to need headphones or some way to keep it quiet. But it's gonna sound great.
I use a THD Hotplate w/ the ValveJr. and it can still get loud enough to be heard across the street w/ the Hotplate at max attenuation when I crank the amp to around 1:00 o'clock through 1 Celestion in a closed back cab.