That's normal for older NADs. Have to just live with it. Just make sure the volume control is at 0 and should not pose any problems for the speakers.
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My old Sansui did the same thing. I placed a couple high wattage resistors in the second set of speaker outs (B). My speakers are on "A". When I'm ready to turn it off, I lower the volume, switch to the "B" speakers that have the resistors and turn it off. The resistors take the load. No pop. When I'm ready to listen again, I switch it to the "A" system and I'm set.
If you do this, be sure the resistors do not contact anything to short out. Place something on the leads if you need to.
Turn off pop could be the result of some excessive DC offset appearing at the output(s). The DC could be high enough to cause nuisance pops but not great enough to trigger the protection circuitry. This can occur in older amps as parts age and values drift. If the amp has offset trimpots, these can be adjusted to zero out the DC.