Something doesn't add up here: if you are of average height, sitting on a standard sofa and your ear is 6 to 8" below the tweeter, then either the designer of the speaker is somewhat detached from the real world or she intentionally put the tweeter above the average listener's ear. In the latter case, either
(a) the speaker design is botched, or
(b) your are sitting much closer to the speaker than it is designed for, or
(c) the effect you are hearing should not really be attributed to the relative tweeter height but to a system quality or synergy issue.
Not knowing anything about your system, I do not want to speculate. What makes me curious is that this thread started about the tweeter rather than an apparant roll off in the highs at your listening position. You should not be able to identify and locate individual drivers of a speaker unless you bring your ear next to the speaker's baffle. Of course, it is entirely sensible to suspect that high frequencies are emmitted by the tweeter ;)