If the fuse doesn't blow it isn't limiting current to the amp. The amp doesn't magically "know" what fuse is in line with it. I could replace the internal fuses in my sub amp with 5A insted of the 30A that it uses and the amp won't care and work just fine until I turn up the volume and blow the fuses.
Assuming that the grounding is good and the speaker connections are correct, a problem with the amp is the logical case.