welp... the best solution is to get an axe fx so you can play with headphones... all other solutions lead to high risk. when a guitar player tells me they were playing quiet... I think "You were playing loud". no shame - own it. if yer a dick like me, I get in front of it, call the landlord and complain that someone is banging on the walls constantly, purposely making no mention of me and my guitar. that said... he knows where you live. the nice guy thing to do is to go over and say - "hey I'm sorry for bothering you. I don't want to cause issues. Is there a time-of-day / day-of-week when I can practice without bothering you?" I'm guessing this ends with him being an ass hat and you being back to annoyed. After all he banged on walls instead of visiting you and trying to reason. I'd ask the landlord if there is another apt # you can move to in the sm building. keep in mind if you do that and still have issues - they are clearly going to see you as the problem (you are, but now they'll see it). I lived in a flat ages ago. neighbor next door came over and went crazy. cursing and random **** about how they are pillars of the community, a professor, bla bla. I called the police saying "of course I'm going to take steps to be quiet but I felt the need to get this documented because there was some serious mental instability there and I'm afraid for my safety". cops went over there. don't know what they said but just the image of cops at your door is something to be avoided, esp for pillars of the community. I put up soundproofing, played a bit softer, and moved out 9 months later. Never had another prob tho.