I've had my 40" since November, use the AMNPS, and have had good results with PB, pork ribs, turkey, and fish. I've often wondered why the MeatProbe (MP) reads about 20degF high and BoxProbe (BP) read about 30degF higher than a $20 Taylor (which agrees within 1F to a Omega thermocouple meter using a type-K probe).
I took the cover off the controller, and figured out the wires to the harness. I think both the MP and BP use thermistors which are probably spec'd at 230-250Kohms at 25C(71F). The top of the thermistors are biased to the supply voltage of the controller (which is about 4.3VDC, coming from the PCB at the bottom of the MES with the relays to drive the element and the LEDs. Basically it's the output of a 7805 thru a diode, why I have no idea). The bottom of the thermistors go to 20Kohm resistors to ground, and the junction point goes to the controller chip (no idea if it's a custom chip or something like an Atmel micro)
I recorded Taylor, MP, BP, and Omega readings with all the probes in a bottle of boiling water that was allowed to cool, and other runs with thermocouples in both the MES air and attached to the BP, and also recorded the junction voltage to the MP thermistor on the controller. I think maybe Masterbuilt assumed that they were using 250Kohm thermistors, but got some that were on the low side; I think I read somewhere that thermistors might have as much as 5-10% error from an ideal curve.
Anyway, I wanted better accuracy in the 140-270 range, so I calculated that 20Kohm resistors to ground should be more like 15Kohms. I tacked 60Kohm resistors in parallel with the 20K to generate 15K total, and now the MP reads 6-8degF high compared to the Omega, and the BP reads 12degF high. The downside is that now at ambient the MP and BP read about 10degF low, but I don't really care about that region.
Here is a picture of what I did:
I wouldn't recommend this mod unless you are comfortable playing around with electronics. If you goof it and short out the resistors, then the controller will never turn off the element, and you'll be depending on the Klixon to eventually cut off the heating. I have yet to read about the Klixon working, but lots of horror stories of the box overheating.
I plan to replace the controller someday, perhaps with an Ardunio.