Not all AVRs are created equally... that is why there is a night and day difference for some and not others. A cheap best buy receiver is obviously cheaper for a reason. Transformers. (insert lazer sounds)
It has to do with the current supply rating of the secondary of the power supply transformer. Voltage is one thing, and does provide SPL. Current is also important as it provides depth, punch and clarity. Dynamics if you will. Even at low volume.
A simple way to think of it is a motor's ability to drive a mechanical load with less current vs rated current. If voltage remains constant and the load is increased current must also increase to stop the process from stalling out. If the current is not on hand, ie. the transformer is limited, then the rate of doing work will slow until the process halts. In a speaker the mechanical load is the driver itself. It's ability to quickly change motion and counter linear forces are directly related to current. Now we can understand why headroom is important.
This is why higher end power amplifier separates quote the kVa rating of their transformers. The higher the kVa rating of a transformer, the more current it can theoretically supply if voltage and frequency remains constant. Ie, the rate of work possible is much better.
This is also why subwoofers have dedicated amplifiers. It should stand to reason speakers would benefit from the same ample power treatment. Deciding what is ample is completely subjective. But it can be argued more real power is definitely a good thing, and therefore costs a premium.
