I did some measurements of the P380 and it confirmed what I feared about it...the Le kills the response over 500 Hz. The FR in the spec sheet is VERY optimistic...basically take the 1 Khz reading and move it down to 500 hz and you have the response. It is a nice woofer for the price if you can cross it over under 500 Hz in a 3 way design. I think I'm done looking for bargains, maybe a Delta Pro or a B&C 15NDL76.
One more side note, there has been some discussion of a 1" compression driver that will go down to 500-600 Hz...if one doesn't mind going 3 way, the Selenium 250-X is cheap, measures well on a Seos15, at least down to 750 and sound pretty good. Couple it with a cheaper super tweeter like the Eminence 150 and you got great response from 500-600 up. And much cheaper than some of the high ends CDs I've been seeing tossed around.
Edit: Nix any idea of the Eminence 150...very erratic FR on 3 different horns. The raw response of the 250-X does indeed roll of below 500 Hz. Now the hunt for a not so expensive horn to match the SEOS down to 500 Hz. Any ideas?