You must be taking about one of those Single Power tube amps? My friend Happy Camper has one. I find it limits the 650 to the point they seem veiled. Love it with the HD800s though.
I live with both the 650s and the 500s every day with a few different kinds of amps. I like the 500s on my balanced M^3 and I like the 650s on the B22. Yes, because what you said about the 650s scaling I find true as well.
I guess a headphone can be both dark and bright as you say - at the same time. I don't find that with the 500s. Laid back a little I say yes to that, bright in the highs and treble - yes to that as well, that's why it pairs well with the M^3. The 650s need a little umph behind it to bright those highs out more that's why I pair it up with the beast.
When both are compared I still find the 500s on the more neutral side of the house. YMMV