If it isnt critical why would a manufacturer put a warning on the back of the unit?
On the back of one of my units it says speakers 8-16ohm only, use of lower impedance speakers will damage the transistors.
No offense and you are a technician but why put it if it doesnt mean anything ?
Even my dad back in the day told me not to drive 4ohm speakers with an 8ohm amp.
If you knew the workings of electronics, specifically power output stages in general, you might understand the tolerances allowable of such things.
But I doubt you know this as I do, so it would be worthless to explain it.