I am also a big fan of the Quatros. I dont think they are as detailed or have a better soundstage than the 2.4s. Overall they are warmer and the stage is set back more behind the speakers. I have not heard any Vanndys that are as resolving as the Thiels. I have yet to listen to any speakers that I like the imaging, and soundstage better than the Thiels or the Quatros and 5As. The Quatro have depper bass but I dont think its as integrated as the 2.4s. Not sure about the brightness you heard. But I would think the electronics played a big part. I have always found the weak link of the Thiels to be a slight hardness in the extreme upper midrange. For me once this is tamed they are the perfect speaker.
I would give a slight overall edge to the Quatros over the 2.4s, but my wife doesnt like them much. "Dull" is her word. (not mine)
The beauty of the Quatros is you can get away with way less power, and have so many more options on amps to use because of the built in subwoofers.
The Thiels (for me anyway) have become a commitment to better upsteam gear. At this point I know if the music sounds really bright its the recording. However, my system is more on the warm side and also slightly forgiving, so this has not been a problem.