I may have missed it among the 140+ responses, but did anyone suggest you run an extension cord from another part of the home to the amp? Perhaps the circuit breaker or the wiring/panel/outlet has a resistance and after an hour of high current from the power amp the thermal resistance is increasing, therefore limiting the bass notes. Notice that the phenomenon took longer with only one channel driven ( as the single channel driving a speaker will draw less current from the wall, but problem still occurred later, nonetheless). Adding a powered sub to the circuit will exacerbate it as another amp is being shared with the AC, no?
3 amps with exact same issue rules out the amps for sure, plus as I used to repair Musical Fidelity, I know the M6i runs very cool (if properly biased), and is Bipolar transistor powered. Removing the cover would be not help if the amps cover is cool to the touch. Plus, I know the M6i is fine with 4 ohms; all day long.
Hope this helps.
Steve
3 amps with exact same issue rules out the amps for sure, plus as I used to repair Musical Fidelity, I know the M6i runs very cool (if properly biased), and is Bipolar transistor powered. Removing the cover would be not help if the amps cover is cool to the touch. Plus, I know the M6i is fine with 4 ohms; all day long.
Hope this helps.
Steve