I've got the same knob and "audio level" face plate! Came off a stereo power amp from the mid 70's, can't remember which always saved it for something better than what I was working on at the time..
I have 2 solutions for attenuation for my 2 amps, when I need to keep the volume down:
My trusty old THD UniValve has their own built-in THD attenuator, so I can use that with my standard guitar speaker cab for tube-goodness at low volume. (The Univalve can also use low-wattage power tubes, including a few NOS tube models that are surprisingly cheap.)
My other amp is 15W and doesn't have an attenuator, so for it I built a second speaker cab with lower efficiency speakers - some good-quality audio full-range mid-range 5" speakers (not expensive, from Parts-Express). Guitar speakers typically produce 98-100dB per watt - i.e., very efficient! Audio speakers are generally much less efficient, the speaker I use is something like ~86dB/watt - so I can crank the amp and still not take the paint off the walls. And now I also use my PedalPCB UniBox speaker emulator in the effects loop, so it sounds like a great guitar cab...
L-pads are to be placed in between the amplifier and driver(s)/speaker, or in between the crossover and driver(s)/speaker(s). Maintains 8 ohm impedance. Wire & Cables. See Auditioning Your Speakers .