[mindonstrike]

I did a google search on your mixer and only came up with 1 retailer and little specs, so it's difficult to give any definitive advice.
I also google searched your line conditioner which indicated that it's just a fancy surge suppressor, no ground lifting capabilities, I may be wrong but thats what they said, however if evrything is plugged into it then a ground loop wouldn't be the problem.
The retailor indicated that your mixer had 2 400 watt outputs. If it's 400 watts into 8 ohms you should be good however if it's into 2 ohms (which would be about 100 watts into 8 ohms) then you are probably underpowered.
You indicated that you are using "a" 1/8 inch mini from the card which would indicate a TRS output (stereo), but you indicated a super hi-z input to your mixer (not sure what super means) which would indicate a TS (mono). You didn't mention whether your cables are TRS or TS so I can't speculate there.
I've never owned a powered mixer so I'm just guessing but it would surprise me if youve got a xlr input along with only one hi-z input.
Also if you've got a setting for +4/-10 db make sure it's set to +4

without more specs it's hard to tell except I doubt it's your computer

Sam