[HayZee518]

the way a speaker is constructed, you have the frame which supports everything. on the back is a permanent magnet of varying gauss. on the frame is a magnet pole piece which is a solid rod. glued to the surface is a mesh spider which centers the voice coil around this rod magnet. the voice coil consists of a fine wire wound around it. it is terminated in a braided wire jumper which goes to the terminals. a dust cover is glued to the baffle front which keeps stuff out of the air gap of the magnet - voice coil. the baffle is a composite paper which forms the cone of the speaker. this is glued to the out side of the frame. when a varying frequency voltage is applied the coil is energized and moves in or out around the voice coil. this movement in and out is what produces sound. if the voice coil is rubbing ANYWHERE on the magnet it will cause distortion. varying temperatures and humidity will expand the voice coil form and cause rubbing. mishandling might disrupt the spider and mis align the voice coil. when a speaker "blows" the voice coil burns out - it opens. some windings might weld themselves together changing the impedance of the speaker and cause rubbing on the magnet piece. turning up the bass temporarily fixed it by moving the voice coil over a large range of frequency but will return to "normal." I don't think your amp is at fault I'd look to the speakers.