I'm wondering what's up with yhour MX60. You didn't post your input/output voltages and currents (real currents), which is very important. Is it that the OUTPUT Ammeter of this particular MX60 is wrong, and are you using some external monitoring ?
The MX60 will have a lower efficiency at low output power/voltage/current. 83% is very low for medium to high power outputs, even 12V systems.
boB K7IQ