I agree with the consensus that processing diferences are not as huge as one might think over a certain price point. after all, we are dealing with a pretty compromised digital process. This is why I found more of a difference on music when comparing units. There, with a proper analogue bypass, the differences can be very obvious. I've also come to be sceptical about analogue bypass, in many cases these do not actually bypass the dsp circuitry at all, just pass through it with the dsp modification disabled. To my mind, there will still be filtering going on within this process.
The power amp will always make a big difference, and I would agree that upgrades should focus on this area. I don't think it's all about power however. Not only can power ratings be misleading, but also one only has to listen to a low power valve amp to hear that a powerful transparent soundstage can be achieved at low levels of wattage.
The quality of power delivery will determine the tonal rightness of music and effects, and give you the feeling of realism, of being there. It will also provide an almost 3D shape to sounds, making them far more distinct within the mix. This, in my opinion , is where most of the highly rated receivers fail to deliver. All of the information is provided, and in the right locations, but it becomes quite confused and fatiguing to listen to.
To use a comparison to music. Many amps will provide a detailed soundstage with lots of individual instruments vying for attention. Far too few will provide a coherent musical whole, within which one is aware of all the constituent parts.
And as for depth of soundstage, again this is where quality counts, as loudspeakers cease to form the boundaries of the experience.