I use the 206E in a 1.3L bass reflex cabinet made by David Dicks (single driver, no crossover or filter). More information at http://www.oldhifi.com/fostex.html (be forewarned, there are a lot of pretty bold claims made on the site - David loves these drivers, although he loves Lowthers more - I chose the Fostex drivers after a side-by-side comparison at David's house in St. Louis).

You definitely lose some low-end with this configuration, and I'm not sure any cabinet or filter combination is really going to change this (David sells bigger cabinets to extend the bass marginally). At the same time, I am really pleased with the overall presentation. On small ensemble stuff (particularly acoustic, jazz, classical) these speakers really sing. If you want really extended bass you probably aren't going to be happy with any single driver (David says as much near the end of his site, recommending Hsu subwoofers for those who miss the really low frequencies). I don't know how low they measure flat, but they usually measure somewhere around "that sounds freaking awesome" using the only tools I own (ears), which is good enough for me. On a related note, you should hear "rub a dub dub, three men in the tub" on the tubes - it's like you are actually there making candlesticks...

Someday (I'm figuring around 2025) I'm going to build some real speakers (7Pi horns) and really rock the Casbah.