HI Jem,
Sounds good! 50Hz is possible if the box is tuned to that frequency, but then you get an upper bass dip (the one note bass). But I have never heard the driver. It certainly sounds like a good one! I am working on a pair of Fostex Fe103A'as now, and preliminary rough measurements indicate a large peak, from 2200 Hz all the way to 8,000 cycles! I'll be doing a notch on that one for sure, a LARGE notch.
-akilesh