As far as the low input impedance I thought this was mainly true with older Class D amps? For example all the recent Bel Canto and Wyred amps advertise that they change the ICE input circuitry to accomodate a wider range of input impedances. Do you think this is still an issue?
I do agree with the power cable suggestion though. The Class D amps do seem to improve significantly when you clean up the EMI. I used a Signature Lessloss cord for this - it really brings the sound into focus. I found other components could make improvements - but similar in nature to how they would make improvements on a Class A/AB amp.
I still think the sound of the BC 500m seems lacking but perhaps its a question of power - I have low impedance speakers with a long run of speaker cable - so perhaps that was the issue. But then again I had tried them out on my older system that had high impedance speakers with very short cable runs and they definitely didn't sound as good as the Simaudio amp. At high volumes it was close, but at low volumes the Simaudio managed to stay very musical but the BC just compressed badly.
Since then I have switched to the Spectron - was not able to compare it against the older system/Simaudio, but so far from memory it provides a very similar sound to the Simaudio, but with more cleaneliness - so I am very satisfied so far.