Quote Hugh Robjohns:
The problems with rules of thumb and generalisations is that there are always exceptions!
Whenever has life been easy! Thanks for the explanation, Hugh. I went back to the ASP8's user guide and, to add a further degree of uncertainty, this says that it's equally fine to use them upright or on their side, the only effect of the latter is to slightly reduce the width of the sweet spot, but not big deal. So I guess it's down to design after all.
No mention of upside-down though and, your considerations about gravity-related issues are slightly worrying, in absence of manufacturer details. I guess sideways will be the safest bet for the time being then...