For sure the response is dropping off rapidly. You need to establish whether its the ECM or the preamp.
As I mentioned before, you'll have to first measure a cable looped between line-in and line-out of your soundcard (and it should return a perfect flat response of course). Then insert the preamp in that loop using the line-in and line-out of the preamp (as shown below)....
UB802loop.jpg
It will have a very good response that will look like this. The red line is the cable and the green line is the preamp....
ub802loop_response.jpg
brucek