The Vox AC4C1 is quite loud (it has a master volume, but according to me it should be cranked, i.e. not be there. It's the whole amp overdriven that sound good!).
I just made this simple L-pad attenuator for it, and it works great (it's just the L-pad and a bypass switch). It does change the sound, but not as much as a turned down master. You can always compensate for missing treble since it's a top boost. With the attenuator you can get it so low that you can hardly hear it, with both gain and master cranked!
If you get a crimping tool you can do this without soldering.
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Did you remove the C20 bright cap from the master volume? The bright cap makes this amp sound thin at lower volume settings and pulling it gives you more consistent tone at different volumes...