I own this attenuator for over a year. I connect with the Marshall Class 5 which has a volume knob to saturate the amplifier. Impedance Selector 4, 8 and 16 ohms. Bypass. Switch 0 dB, +3 dB, +6 dB to increase slightly the highs you lose a little if it reduces too. A big knob 0 to 10 for adjusting the volume. An amp in jack, speaker output and headphone output.
Amp works with from 1 to 25 W max. However, I also use it with a Marshall 50 W without exceeding the 4 volume amp.
Loss of sound quality up to 2 or 3, then it's great.
I bought used 50 euros, I'm really pleased. I push the volume up to 5W of mouth very hard substance.
If you find a used one, please. I disassembled it seems simple to me, no PCB, 4 or 5 components. Links on the Web explains how to make one for less than 30 euros.