I had the 500s and 6s together on the same speaker amp and come to the conclusion that the 500s were about 85-90% of the 6s. The extensions on both ends are better defined and the soundstage was more expanded. But that could have been placebo on my part. Seems everything preferred around this place is placebo.
Those who know everything about how things sound from reading the spec's and measurements and how you can't hear any difference etc. etc. Well, I stay clear of the science forum for that very reason