Make sure you have installed the latest version of Universal Control and performed a firwmare update. On the Mac, the 1818 can be use class compliant right out of the box, so if you haven't installed UC, you may not know there is a firmware update which has addressed some input gain issues with earlier firmware versions. The factory firmware is an older version, the update in UC will bring it up to date.
The preamp inputs have 60db of gain on them, so there should be plenty of room for you to work.
Some devices need to have pre-amplification before going into the interface. If you find that you have to crank the input on the 1818 to get enough signal, consider using a Direct Input Box (DI Box) to boost the signal before it goes into the Pre-Amp. Most keyboard have their own volume output so this shouldn't be necessary, but the impedance output of guitars and other instruments may not be enough and a DI Box is needed.