Hunt Amplification will install a Master Volume in almost any amp with room for a spare knob for a flat rate of $60.
The Master Volume I choose to install depends on how and where you use your gear, your desired tone, and the amplifier circuit design. Generally, I use one of the many PPIMV (Post Phase Inverter Master Volume) circuits, but sometimes a pre-phase inverter master control is the best choice.
If your amp already has a Master volume but you do not like how it performs, I can most likely replace it with a better sounding circuit.
For most amplifiers under 50W, I recommend choosing a Power Regulator circuit instead of a Master Volume control.