What you learned in that sound class is certainly correct. If you set the levels as high as possible without overloading, you'll get the cleanest sound. If your Zoom recorder hovers around -36, it's badly underrecorded. Even that is still better than overrecorded, but for the future you want to make sure you set levels properly.
Boosting gain later will introduce noise, as you noticed, and the amount and type of distortion is dependent on the recording format. 16-bit still sounds great to me, but does not take kindly to boosting by 30db. For live audio recording, 24bit is safer. A margin of 12db is no problem.
The fact that your iPhone recording is louder is probably because it sets its levels automatically. This gives you a nice workable level, but will destroy dynamics. In a concert situation, this is to be avoided. In an interview situation, especially if you don't have much time to set up, it is acceptable.
Stefan
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macpro 2x3 ghz dual core intel, 10.6.8, FCS 2