Wayne, the high level of technical complexity of your drawings did in fact require considerable study to determine why you were complaining. What can be said at this point is what the problem isn't: connecting a voltage source in parallel to two loads doesn't create a "voltage divider" in the technical meaning of the term(a series connection does that); each load receives the original voltage. For example, if the sub out from the receiver at a particular instant is 1 volt, each sub amplifier connected in parallel receives 1 volt(not a half volt).
That's all very well, but the question of course is what the problem is, then. It appears that both branches of the "Y" and both coax cables are working separately, but didn't work together in "E". This makes no apparent sense. Although the amount of time that you spent indicates that you probably double-checked the connections carefully, I'll nevertheless suggest one remote possibility which afflicted one of our members a few years ago: in making cable changes he inadvertently plugged into the multi-channel sub input on the receiver rather than the sub out. Are you certain this didn't happen when you ran "E"? If this isn't it, I'll continue to ponder the matter, but my next best guess at this point would be Fred's suggestion that you ran out of Boom Boom after "D".
