The ELF is a circuit that RJW created that combines with certain BE cabinets, which are sealed instead of vented. It extends the lows well beyond what the cabinet would normally be able to reproduce. I've never used one, but I've heard that it's a pretty phenomenal way to go as far as getting really thunderous lows out of a modest amount of cabinet. I've always thought about checking an ELF rig out, but have never had the chance.
Your 18s should be able to crank out the big lows pretty easily. You might want to experiment with stacking configurations. I've found that coupling the speakers in various ways can vastly influence how the lows project. Having stereo is very cool, but in my experience, stereo low end can diminish the impact. So, you could consider putting the 18s on top of each other in the middle and then spreading out the high cabinets. You could then put the rack on top of them and get it up to eye level for easy tweaking. Given that lows are pretty much omni directional, you'll probably gain more than you lose.
I've managed to get some seriously thunderous lows out of my F2B (or SMS Classic)/Crest CA9/Euphonic Audio CXL-12 rig. With just two of those cabs, I played an outdoor festival last night and it was pretty damn thunderous. Definitely shook my side of the stage and the sound crew (some of whom mix in the largest rooms, ie. arenas, etc. and festivals out there) were pretty amazed at the amount of oomph. Not bad for two 12s! However, I've been thinking about dragging out the 18s and 15s again.
