Thomas,
Two things. First, it's easy to demonstrate this affect by simply tilting you're head down. HF goes up, for me at least.
Second, I use PMC MB1 XBD floor standing monitors. For proper time alignment (which is easy to verify with pink noise and a bit of head bobbing), PMC specifies that the mid driver be at a 10 degree angle up from the listeners ears. At first I found this a bit strange as the image is slightly elevated, but now I find it completely normal. Imaging is outstanding.
What are your thoughts on their approach? Have you every used either the MB1/2 or BB5 in an installation?
Thanks,
GR