Mick, I have tried my Larcs in a several different positions, on the floor as the OP here has his, I have also had them-and somewhat prefer mine-in my current room, on stands that are heavy/solid, and have a top plate foot print of the exact dimensions of the Larc base, and standing upright. I have also had them on the same stand, only laying on the longer dimension/back of speaker, firing much like an elongated mini-monitor would. That too was an interesting listen, and actually very good too. I think ideally(?), to get the proper bass end, it is important that the bottom base of the speaker is on something solid, so that the friction loading(or whatever one would call the multiple tiny ports/holes in the bottom) of the cabinet works as intended.
But as a true bookshelf siting, I have not tried that, as I never really had a bookshelf that would allow such positioning, so I really cannot say. It would seem to me that depending on which way around you would situate the cabinet, it could impede the drivers from doing that open/semi omni thing. I am all for experimenting with speaker positioning though, even oddball settings that fly in the face of wisdom and convention, sometimes just works. It all comes down to the ears of the beholder in the end, and there really isn't such a right or wrong way with a lot of this stuff. FWIW.