Also are you sure that the R-S500 is the same as the A-S500 because they is quite a tidy price differential there I can't imagine adding in a receiver unit would drop the price like that. Am i wrong in assuming they might have cheaped out on the innards of the R-S500 as opposed to the other?
You're probably not entirely wrong. I have no firsthand experience with either, so I can't say for certain. But
I have read that the integrated amp has a beefier power supply than the receiver. Be that as it may, I'm highly skeptical that changing your amplification would have any but a placebo effect on the sound stage and imaging of your speakers though. There'd certainly be a more profound result from moving the speakers out to a wider angle, and playing with toe-in-out. I still say if you've got money burning a hole in your pocket, buy a measurement mic and play around with Room EQ Wizard and EqualizerAPO.
You might also consider the possibility that the lifelessness is in the program material. If you're listening to net radio streams, find higher bitrate streams. If you're looking for plausible imaging from hip hop, switch to jazz. etc.
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Good speakers such as yours often reveal flaws in bad recordings.