How do I deal with this upstairs neighbor issue?

I live in a very old apartment building with only 4 units, where noise transfers pretty easily. My landlady has been very ill, so I worked with her to show and rent the apartment above me when it became vacant. I had a few prospects, but found a very nice, younger couple who seemed professional and quiet.

During the showing, the guy mentioned that he was going to school for guitar. When I made a concerned face, he assured me he would not practice from home. I said okay, and iterated that if he needed to practice here, this wouldn’t be the best arrangement for them and it would be best not to rent the place. I explained that at least a few tenants worked from home. He told me not to worry because he had two other places to practice.

Guess what? It’s the third week they’ve been here and he’s been practicing loudly, every day, almost all day on his electric guitar. It’s so loud that it sounds like it’s in my livingroom, which is a problem since I work from home. I wrote a nice email explaining that the practicing was an issue, that myself and people on the other end of my phone could clearly hear it, and that my understanding was that he wouldn’t be practicing here.

They wrote back saying that they would try to add some egg crates and cut down on the noise, and apologized for disturbing me, but they basically told me that he would continue to be practicing from home, frequently. They said they’d be ‘conscious’ that I was below them. In other words, too effin’ bad.

I’m not quite sure how to handle this. Because of the closeness of the apartments, I really need to have a good relationship with them and I don’t want to screw that up – but this situation pisses me off since it was clear that he had planned on practicing here all along without regard for how this would impact others.

Any advice on what I can say or do while still being diplomatic and not coming across as a total bitch? I want to call them out on not being honest about their intentions, and I also want to tell them that there’s a high likelihood practicing here will not work, but without damaging our relationship.