75db is fairly loud in an apartment. 80db would likely upset grumpy neighbors. 85+ would likely p.o. even nice neighbors. Over that and you should expect an eviction notice on the door.

Or have I just had a rash of bad luck with apartment neighbors?

Some nice hifi shops will let you demo units in your home. You'd likely need to leave a heft deposit, and some form of id, but it would certainly be the best way to know if a Denon 2805 made your M80's sound better.