it refers to the the perceived "harshness" of the treble in speakers.
Brightness as originally defined, and as Dean is using it, refers to an 'edge' or crispness that you get with live music.
Generally people like a slightly rolled off high end, but if you look at anechoic measurement for Paradigm studios, or Axioms, you will see that they are reasonably flat over the upper end of the frequency, and there is nothing in the measurements that would indicate that either brand would be 'harsh'. It does give these speakers a sharper/crisper/edgier sound.
That said, at this point, you need to listen to some of these to decide for yourself what sound you like.
With a $5K budget, I would look at allocating a nice chunk to a good sub that will reach nice and low for movies.
Now, collect some well recorded music that you are familiar with, pick up a notepad, go forth, listen and record. Your ears will narrow down the selection for you. Have fun and if you buy something don't forget to come back and post lots of pics.
