Hello Jim,
I don't know about the slope part of the brightness adjustment, it does affect the entire overall brightness and is quite noticeable even slightly moving either the entire master slider up and down or each individual slider per pipe. I liken the brightness control to having very similar effects that a treble control would have such as you'd see on an AV receiver. I use the brightness control either if I feel the pipe or pipes sound dull and lack sharpness and detail, or are too bright sounding where I can then dull them down some, again very much like a treble control. Try sliding the controls up and down while you play the stop selected and you will easily be able to hear the effect it has, you can go from mild to wild if you like.
As far as the hi / low EQ goes, it seems to have a much more subtle and narrow affect on the overall timbre, sometimes I can't even hear the difference, so I generally do not use that control and rely mostly on either the brightness or amplitude controls for voicing. The tuning control can also have some interesting effects.
Perhaps someone here can dive into greater detail as to the effects and when / how to use the hi / low filters.
Marc