Reply to switching from vci100 to s4 problems

Hey DJTT! Just got my S4 and having a couple issues. I'm getting low output from the S4 as opposed to what I got from my VCI100 + Audio2DJ. It sounds really flat and the bass is hardly there. When I use my A2DJ instead of the S4 sound card the problems go away. I want to use the s4's card instead of lugging a A2DJ around with me.

Does the s4 require some kind of amp or something? I'm using it as an internal mixer just like I did with the VCI. Any help would be greatly appreciated!