There is nothing wrong with hooking up a 70V loudspeaker to a low impedance tap on an amp. Except it won't be able to get anywhere near its full rated power-unless it is a 600 watt 70V loudspeaker-not to many of those out there.
As JR said-measure the impedance of each of the lines and note where they were attached.
It all might make sense-when you look at the total impedance load on the amp-or maybe not
.
Anyway-you have to ask yourself "What am I here to do". If the complaint is about the buzz-figure that out-I doubt it has anything to do with the weird wiring on the back of the amp.
But it could have to to do with some "other" wiring errors-such as a loudspeaker conductor touching a building ground and so forth.
Unplug all the inputs to the amp and see if the buzz is still there. If it is not-then go about finding the culprit.
If it is still there-then I would remove one loudspeaker pair at at a time (you notice I said a PAIR-not a single conductor of a pair) and see if the buzz goes away.
See if it is localized to a particular wire or not-or the unit in general.
If the amp has been working for awhile like it is-it will probably continue to work.
If the unit is faulty-then I would talk to the customer and tell them what is wrong and what it will take to fix it RIGHT.
I would either do it right or walk away. I would NOT try to patch it up-no matter how much the customer wants you to-it will be a headache that you will never get rid of.
Let somebody else deal with gum and twine repairs and the consequencies they bring