The line inputs on your sound card are
meant to take line outputs from other sources (such as the empeg). That's why they call them "line level"- they are all the same. So it is OK to plug your Empeg into the line input-- that's precisely what it was meant to do.
Speakers and headphones are not "line level", they require power, and that's why there is the note in the manual. (Self-powered speakers are another discussion entirely, but we won't get into that right now.)
Currently If I put the empeg at about -9.0db volume and the line-in input volume to the max I have to put the wave and CD inputs to nearly the minimum to achieve the same volume on all inputs.When you are plugging the empeg into the line input on a home-stereo or a computer, you are supposed to set it at 0db. Try that and see if it helps.
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Tony Fabris