The 175 requires 802.3at to be fully supported, as some of that power is required to run the heater. You will sometimes see the AP-175 come on and boot up fine with 802.3af if the heater is not required, but once the heater kicks on if/when the temps drop below freezing, the AP will shut down and not turn on until af power is established or the chassis gets above freezing.
So better to be compliant and use 802.3at power.