All three are designed to make the most continuous power at 4 ohms bridged or 2 ohms stereo. They will make slightly less CONTINUOUS power at 2 ohm bridged loads, but PEAK power increases at 2 ohms bridged. The lowest impedance you should ever run on a ZX, XS, or M series amp is 2 ohms bridged or 1 ohm stereo.
Phoenix Gold knows that people will connect these loads to the amp so they build them to take it. It's important for each design to continue playing even when abused. However, there are limits to how much power each design is allowed to make.
The design must be able to dissipate the heat generated by the power it produces. If you can't get rid of the heat, the amp will destroy itself or if properly designed, thermal off before anything breaks. So the M, ZX, and XS series amps all have circuitry that limits the power output (and heat stress) for any musical signal longer than about 50 milliseconds.
Now it's important to understand that musical peaks are much shorter than 50 ... more.
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