i got a Hartke A70 for a few days, but something coming up. there's a 'brrrrrrrr' sound when i play some low note, then i check the speaker on the surface (front cone) and find no cut or broken at all. then i check behind speaker from a hole to make sure that there is no spider web, dust or something else and i found it is pretty clean i'm afraid that something going on inside the speaker cone, any suggestion for this? for the worst possiblity, if i replace the speaker (for a better one maybe) is it worthed? the speaker spec is 12" with 8ohm, so what kind of speaker will fit for this amp?