I'm not sure exactly how you're hooking things up. Presumably, you're not plugging both a bass and a keyboard into a single jack. I believe (without being quite certain) that that unit has a number of standard-issue mic/line inputs, and another input that has a line/guitar switch. Assuming I've got that right, and that you want to connect the keyboard to one (or more likely two) inputs, and a non-active bass directly to another input, do this:
- connect the L and R outputs from the keyboard to two ordinary Mic/Line jacks.
- plug the bass into the jack that has the Line/Guitar switch and put it in the "Guitar" position.
As an aside: it's more a matter of impedance than level, I think.