I just looked at your amps, not drilling a hole and adding an alps volume control to those beauties.
I still would prefer to gain something while solving the issue, but it would have to be a separate box in front of the amps.
My McIntosh mc2250 had individual L/R volume controls; I added volume control within the case of my antique Fisher 80az mono blocks. Playing straight into those amps was always a treat. Just had VAS fix some Dynaco Mono blocks for my friend, plenty of room inside, adding volume control would have been a good idea now that you make me think of it.