Well actually i wonder how it could be whithout a sub and adding a pair of m60 instead of my M3 as front.
My suggestion is to play something that is to loud in his house but just through your M3s and see if that bothers him. Then play a test tone of about 30-40Hz through your subwoofer (simulating the bass extension of the M60s) and see how loud that is for him. My bet it the M60s will be loud enough in his house to still be bothersome.
One reason I often suggest bookshelf speakers and a subwoofer for people in apartments is that you can always turn the subwoofer off at times it’s bothering them, but still occasionally use it like when they are gone.
Late at night in my apartment I often switched to headphones if I was listening to something like a movie or video game I wanted really loud.
