What do you mean by feedback, exactly? Feedback to me means that output from the speakers is getting picked up by a microphone and "feeding back" into the signal to create an infinite loop of screeching. Seeing as I don't think you've got the microphone hooked up...

If you're experiencing distortion, try turning the empeg volume down which at some point will make the 4v outputs into 1v outputs that your cd player is expecting...

Is the engine noise(pitch goes up with acceleration) you're hearing the same when you're listening to a CD and when you're listening to the empeg? Is it empeg only? If it's empeg only, check the empeg's ground, then try connecting the sled to ground also... Do a search for boxter installs, as they're the ones who usualy deal with engine noise coming from the empeg...

Matthew