so I have a fairly new Ac15 with a problem. When I turn it on it sounds fine but after it warms up a bit the sound gets fuzzy and starts breaking up almost like the speaker cone is torn but it's not because it sounds fine when it starts. Could this be a tube issue? I'm pretty sure it's covered under warranty but has anyone had experience with this. It's really not playable after it warms up and the top of the unit gets really hot.