"The Marathan/CV/Crown will do 2000-2500 watts per..."
just to clarify. that is 2000-2500 watts per channel into 2 ohms. dual ho's per channel would net 1000-1250 per driver which is pretty much as high as you want to go in 4 cubic foot sealed enclosures per driver.
"So then I could build two dual opposed subs. Use biamp binding posts. and power them with one ep4000?"
yes. wiring the drivers in parallel would net 2 ohms per enclosure. no need for bridging, just hook one dual opposed enclosure to each channel. that will give about 450-500 watts per driver.
if you get a second ep, then you will have one driver on each channel of each amp at 4 ohms. iirc, that would get you around 650 watts per driver.
the reason the Marathan/CV/Crown works a little better is that it puts out 2000-2500 watts per channel at 2 ohms, so it is a better match for the drivers.
but, 450-500 watts per driver into 4 18" ho's isn't like a beginner system. depending on how large your room is, you might hit full reference all the way down. and with a little room gain, 130db in the music range isn't out of the question.
doubling power at that point might pick up 2-3db, just for perspective.