Full range sweep - odd graph

I've just been having a go at doing some full range sweeps, with a view to producing some convolver filters for use in J River. The graphs though look really odd - there's a 25db drop off from 7k to around 250. My speaker should roll off at around 55, so I can only assume I've done something wrong.

I'm measuring with a Behringer ECM8000 mic connected to a Behringer MIC100 valve pre-amp, and an M-Audio Audiophile 2496 soundcard. The sound card calibration looks 'normal' i.e similar to others on here, no mic calibration file.

Any suggestions?