I've been the victim of this. I thought it would be handy to just run the cable along some metal HVAC ductwork. System did not sound right. Replaced the cables on a whim (with the replacements just lying on the floor), and all was well. Rerouted the original cables and all was well.
I should think that running a cable in or on metal is akin to using shielded cable - much more capacitive than the norm. The more capacitive the cable, the lower the impedance at high frequencies (and on transients - you have to charge that capacitor). Many amps have an inductor in series with the output to counteract the capacitance of the load, but apparently this is often not enough.)