It sounds like possibly a capacitor is letting go. They stopped making the 1900 almost 40 years ago so it’s way past time for service. If the amp had been sitting around not being used and you fired it up this can be a problem. I think the speakers are 8 ohm but if they go low at any frequency I know the amp is not happy at all. I liked McIntosh integrated amps but there inability at low impedances was a deal breaker.
I see that you're looking into an insurance claim but when you buy something as old as this amp it's as-is unless he gave you something in writing.
I see that you're looking into an insurance claim but when you buy something as old as this amp it's as-is unless he gave you something in writing.