I'm reluctant to use amplifiers rated for more than 100W in this application. Likely, the amplifier has a small DC offset, perhaps only during startup, but this DC offset causes the volume control autotransformers to saturate. Once saturated, the autotransformers present a very low impedance to the amplifier and it will misbehave. That "knocking" sound that you hear from the speaker selector is generated by the controls as they saturate.

There are some not so good options: Start by measuring the amplifier output with a DC voltmeter while no speakers are connected. If you measure more than about 0.1V, be very annoyed. You may be able to adjust this -- at the risk of voiding the warranty. Another approach is to add a small series resistor between the amplifier and the Niles box. Probably, something on the order of 0.5 Ohms (plus or minus) will do the job. Pay attention to the resistor's power rating and allow some air circulation.