Combing does occur with the cab horizontal or vertical, it happens anytime two or more drivers are side by side. It's just worse the more drivers you have side by side. But combing is not the major issue anyway. That happens at frequencies high enough to not be a concern in most cases, though this would be a worst case scenario. The major issue is the dispersion angle of the mids, which is inversely proportional to the width of the source. That means an 810 laid down has half the mids dispersion of one upright.