I would say treble is actually one of the strengths of the Quad. I also have an old version of the Stax Lambdas which excel at treble and the Quad compares very well. It has a good energy/extension in the top end without ever being aggressive like dynamics can be sometimes. The only thing is that it's not 100% open like the Stax or like my HD600, it sounds more like a semi open headphone. It doesn't mean that it sounds closed in or anything, but you still get the feeling of having a headphone on your head and being isolated from the outside environnement. Which can be enjoyable depending on the perspective.
I'm not sure about ambient but for anything like trance, dance, etc, the Quad is crazy good with its combination of deep bass, sharp treble and speed.
Thanks, this is really encouraging. I listen to a lot of dance music as well.