Topic: Do I need a preamp?

This will, I hope, be my last cry for help. I recorded speech signals using a pair of microphones AVENSON (connected to front analog channels 7 and 8 in FF800). By deploying oscillogram of the signal obtained, I observe the problem I've been talking about for one month that is no regular distribution of energy in relation to the axis, "0 point" or reference line. The signal shown is, some times relatively above the reference line, and in other instances, relatively lower. This problem of signal distortion obviously is not noticed just by listening or deploying another type of graphic, as a spectrogram, for example. But, with the oscillogram, which is the minimum and the basics, the situation is serious.
I recorded with the same microphone on a TASCAM DR-100 MKII and used the same software for graphical display of the signal and I have no problem. So, will I need to work with the preamp when using a FireFace 800?
Thanks again for your help.
Lorena Ham.-