Low sound using 1/4" rear inputs

On the official User Manual, you describe a scenario using a 1/4" Drum Machine in 7/8 rear inputs.

But I noticed that 1/4" inputs come with a very low leven and cannot be adjusted with gain knobs (as the manual says).

Is there any way to compensate this? The sound comes very low to Cubase.

I'm running Win 7 with Cubase 5. Any help will be appreciated!