Hi Tim,
Unfortunately I can’t answer your question relating to the lowest voltage that a MX-P50M will run on, but it would be interesting to test this out. From experience, the normal operating range of a 4S LiFePO4 is fine for the amp and I certainly wouldn’t want to take the battery beyond its voltage plateau. Three hours on air last week only required a little over 2AH to recharge my 4.2AH LiFePO4, but like Paul I usually run my amp at 25W output (2.5W drive) to conserve battery power. Reports on air have been excellent at both 25W and 45W output levels.
To date I haven’t found a decent review of the MX-P50M which is surprising seeing that the amplifier has been around for a number of years now. The design of the package is obviously based on the 13.2V standard for convenience with it being designed primarily for portable use. The physical size of the heatsink is undoubtedly a limiting factor, particularly as the efficiency rating of the MRF186 is specified at 30%. The spec sheet gives an output rating of 120W PEP on 28V at 960MHz, so the device is being conservatively run, but it still gets quite warm running at 25W output.
73, Gerald