Brandon, your results in general reflect the real-world realities of amplifier power use, which are often imagined to be far higher than actually required in typical home use.
No, the 75 watt max reading above the 48 watt idle reading doesn't mean that only about 25 watts of additional consumption are being used for maximum speaker output. The watts at idle are being "wasted" to keep things ready. When the speakers are actually being driven, that power that was used at idle is put to use in actual acoustic output. So, much of the 75 watt consumption(some goes for auxiliary circuits other than amplification)is for amplification for the speakers.
As to the observation about the sub consumption remaining at 28 watts, that does not compute. If the sub is actually outputting a high level of sound(are you sure?), more power consumption above the idle level will sometimes be required.
