Incidentally, when I started this thread ( http://basschat.co.uk/topic/134869-re-valving-an-orange-bass-terror/page__hl__otb%20valve ) I ran my OTB500 from the DI into a mixer, took a feed from the mixer into my Zoom H2, and monitored using headphones.
Not a speaker cabinet of suitable impedance in sight.
I suspect that, as others have said, this warning is of the "may contain traces of nuts" variety.