To support my hypothesis with words from my google box:
“ The reactive load imposed on the amplifier by a speaker load causes far higher dissipation in the output transistors than the simple resistive load generally assumed during testing. At the extreme end, consider a load which is completely reactive (i.e. inductive or capacitive). The voltage and current are 90 degrees out of phase with each other, and no power is consumed by the load - even though there is voltage and current present (and measurable). Assuming a voltage of 20V and a current of 2A, the actual power is zero, so the amplifier must dissipate not only the normal internal losses inherent in all power amplifier designs, but the 40 Volt/Amps reflected back from the reactive load. (Volt/Amps - or VA - is roughly equivalent to Watts - but only when the load is resistive, implying that work is performed).
In reality, the reactance is always accompanied by some resistance, so the amount of power converted into work (moving the loudspeaker cone to create sound) will always be non-zero. An additional quantity of the supplied voltage and current are converted into heat (another form of work) due to resistive losses in the voice coil and crossover network. The reactive (also known in electrical engineering as the imaginary) component is reflected back into the output of the amplifier, where it must be absorbed and converted into heat.
It is the reflected power from the loudspeakers which is responsible for a great many amplifier failures. Because of the low efficiencies of most modern speaker systems, more power is needed from the amplifier. This means that the amp will have to dissipate more reflected power and this can lead to overheating (or internal "hot-spot" localised heating) which leads to the destruction of the output transistors. Some amplifier protection systems are sufficiently sophisticated that they can prevent this form of damage completely, but will generally provide an additional side-effect - the deterioration of sound quality. This is often noticed as a "grainy" or similarly described quality to the sound, and is difficult to eliminate when protection is used. A certain IC power amplifier I have tested has a very comprehensive protection circuit, which seems to work very well. However, as the amp reaches the point of clipping, the distortion component is multiplied tenfold by the protection circuit, with the result that what should be completely inaudible distortion becomes very audible indeed.”
Transistors are probably at fault. They don’t usually look bad.