Do a load test with existing 70v speaker runs before jumping on a new amp. If the load test is suspiciously high, or suspiciously low, check all the 70v speakers. Make sure nobody has hooked up a plain old 8 ohm speaker in a back room or custodians shop. That could cause most of the symptoms you're reporting.
Years ago I was troubleshooting low volume on a car dealership paging system. I heard a page in the ceiling in an office in the service dept. - asked someone about it and he said they took a speaker from a car and wired it to the paging system - directly on the 70v line. It was actually a 4 ohm speaker. There was a big pillow wrapped around it to reduce the very loud level.
