Use SONAR to record
music, not
audio.
If a part doesn't contribute to the song as a whole, mute it.
Mix at a really low volume and get it sounding right. Then it will sound
fantastic when you crank it.
Test your hearing at least once a year. There are preventable causes of hearing loss, and testing can uncover them.
One well-played part will have more impact than two layered parts. Of course, you don't always want impact (e.g., pads).
Realize the QuadCurve EQ has four very different curve characteristics. Choosing the right one for the right job makes a big difference.