i just got an 18i6 and hooked it up to my mac (osx running logic 9.1), and am getting a very low input level when connecting my roland vdrum kit via any input (line 3-8 in the back, or line/mic 1/2 in the front). if i crank the volume over 50% i am able to hear the audio, but the input level is extremely low. i looked at mixcontrol and everything appears to be correct, and the level out of the roland kit is at full volume. also i'm getting a hum on inputs 7&8 in the back, no clue why that would happen. any ideas as to how to increase the line input volume? i tried the gain knobs on the front and nothing happens. both the daw and mixcontrol show the input, but it's coming in around -34db.

Hi there,
I am new to the 18i6 too and yes the input signal is lower than anything I have personaly worked with. Apparently it was done on purpose.
When I was testing it, I used my keyboard directly pluged in at the back (channels 3-4). I had the volume of the keyboard all the way to the max and I was getting around 25% of the volume I was actually expecting.
For now I have pluged it in the preamps at the front and clicked on "inst" on the Scarlett mixer. It sounds "normal". 
I suggest you do the same.

As for the hum on 7-8 I dont know what to tell you. Does the mixer show any signal even if nothing is pluged in?

Good luck!