FordPrefect, I had a similar issue as yourself, and did the same trouble shooting that you did.. in my case i wound up turning the SUB gain on my processor up a considerable amount, maybe from 3->8 or somewhere in that range.... i then adjusted the gains at both subs one sub i had to turn the gain up, the other sub i had to turn the gain down...
When i was trouble shooting, i went so far as to pull the amp from my EP600, thinking i was having an amp problem again..... when you have both subs in parallel, have you placed your hand on the driver to see if there is any movement?
How are you verifying "Nothing Worked"
The way i eventually discovered i just needed to adjust the gain was to turn the volume up while placing my hand on the driver... When the drivers were moving just a little bit at a louder than normal volume, but not audible i knew they were A) They were receiving a signal
B) not receiving enough signal.
My EP600 is about 13-15ft from my processor, and my JLB is about 6 ft away.... i am using a Radio shack Y splitter....
Hope this helps.