Was the purchase made as used items? Because it sounds like an internal electrical issue with the amplifier. The inductance of the low pass filter is connected still in parallel with the full range circuit, so what it sounds like is the inductor or a component in that phase of the circuit within the amplifier is not functioning correct. Its likely not the wiring of the drivers, unless the issue started after intinal operation....where I would ask, are the drivers connected with too low of an impedance for the amplifier? This would cause a manfunction of the LP network where it tried to work too hard to get power that it didn't have. This is less of a possibility with higher frequencies. So, it sounds like the amplifier was misused before. Without knowing anything other than what was mentioned initially, this would be my first assumption.
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Also, is the amp a 2 channel amp?