which sound card?
did you check the settings of the recording mixer in the computer?
that mixer has noisy preamps when you push them too hard
(to compensate for attenuation elsewhere in the chain, probably..)

mic input gain needs to be set correctly (i think thats explained in the manual)
the faders levels can be at unity gain for a single channel (0)
the internal mixer probably has some kind of trim control and source selection,
and cubase needs to be set up properly.

if your gain structure is incorrect, you might get distortion in the mixer (one of the levels is too high),
and then trim it all down way too much when going into the computer, for example.
the result will be a low level distorted sound.