Question 2: In this example, is…
“decreasing bass -6db, mid -3db, leaving treble 0, then setting volume to -18rms”
exactly the same as
“decreasing bass -3db, leaving mid 0, increasing treble +3db then setting volume to -18rms” ?
Theoretically, yes it’s the same. In the real world, you might get differences depending on the design of the equalizer.
Question 1: In regards to EQ adjustments I’ve always read decreasing is better than increasing (for less distortion). Is this true?
If you are working with a digital audio editor (like Audacity) you can avoid clipping (distortion) by adjusting the overall level before you save to an integer format (such as regular 16-bit or 24-bit WAV). You can boost as much as you want temporarily, but you may need to bring down the overall level before you render.
The Amplify effect will default to whatever change is needed (up or down) to set the peak(s) at exactly 0dB. After equalizing, it’s a good idea to at least check your peaks by running Amplify, even if you don’t end-up applying the effect.
Cut, rather than boost is still a good general rule. But, there may be times when it makes more sense to boost.
The -18dB RMS level doesn’t relate to clipping. If the peaks go over 0dB, you are in danger of clipping. Some files will clip at -18dB RMS, and some won’t depending on the peak-to-RMS ration of that particular recording. (Regular WAV files files are limited to 0dB, and you analog-to-digital converter and digital-to-analog converters are also limited to 0db.)
Note that equalizers can sometimes can sometimes end-up boosting the peaks when you only cut a frequency band. This is due to different frequencies getting phase-shifted differently (a side effect of filters/equalization). Basically, the peaks from different frequencies get moved-around a tiny bit, and they add-up differently after EQ, which makes some peaks higher and other peaks lower.