Thanks for your response. Your question is fair, and I don't have a clear answer to give. As both you and I (in my initial post) have suggested, there are several other variables that may well be responsible for my not being quite as satisfied with the current system.
Of course it would make sense for me to try the GMA speakers before switching cables, but the combination of the hassle (they are matched with very heavy Osiris stands, etc.), and the very attractive openness of the Harbeths, lead me in the direction of perhaps trying a different cable instead.
Oddly enough I can't really say that the Davids are lacking in anything in particular, so I suppose I am curious about opinions on other excellent cables that are relatively inexpensive on the used market.
What I can say, however, is that I tend to emphasize the importance of accuracy, imaging, mid-range and highs, and am not especially concerned with the very low registers.