I personally feel that it would be the speakers!!
Today, most speakers will have the unmeasured 8 Ohm stamped on them even before they decide which size hole to drill in the cabinet.... and some unscrupulous manufacturers even go as far as quoting the classical freq range of 20 to 20kHz for good measure.
If the speakers are less then 8 Ohms, say 4 Ohms then the poor amplifier has to put out double the current ( well almost, for this simple comparison case ). Everything in the amplifier is stressed, including the power supply. Do you get any sort of clipping effect? By that I mean the sound from the speakers begins to distort audibly, even to the unseasoned listener this phenomenon is obvious.