The one possibility I may have missed based on having read what 77PBass wrote: The amps might merely be "up there," as opposed to actually doing any meaningful work. I mean volumes set super low. In that case, the delay-related problems they'd cause might be tolerable.
Another possible scenario I might have missed: Maybe the sound man is delaying the mains by X milliseconds to compensate for the distance between the 360s and the mains' bass bins. Doing things like that is a piece o' cake these days. More I think about it, that option seems to make the most sense. Betcha that's what they're doing. It's still got some rough edges to it, but it might be workable.
Like your say, there are various means to keep things under control. My take on this matter is that the musicians primarily rely on the IEM:s and wedges. Since IEM:s don't flap your pants, the bass amps are likely run at moderate/normal levels -- I'd say in the 100-110 dB bracket -- to add a little tactile stimuli. This is of course enough to be heard from the front row, but not enough to cause significant interference with the FOH running 20 dB or more louder.
That the 360/361:s are used to fill the house is out of question today IMHO. Technically it'd be easy to control their volume form the main desk, but the 70:s are over and one generation of half deaf rockers is enough...
