Hi, Rick.
Actually, the statement below,
There is generally no point in removing absolutely all the noise if it is masked by frequencies in the actual audio, as over doing noise reduction can produce extra unwanted artefacts and can also sound unnatural. If large amounts of noise removal are necessary it is best to do it a little at a time, taking new noise profiles as you go, rather than all in one large lump.
in my experience, I have found what you said to be false with regard to Hum and harmonics, noisefloor, and general EQ issues. Sporadic, spontaneous anomalies should be fixed during a mono QC / remastering pass, but continuous anomalies throughout should be processed at the very beginning and throughout program. Spot de-noise is going to end up sounding choppy and inconsistent throughout. As you may have noticed, my settings were pretty conservative, and yet the playback yielded a much-improved sound. For the Hum and EQ, I would use an insert plugin to address those issues, that way, you can experiment non-destructively.
For the broadband de-noise, I would process on a duplicate file of the original and save as such. You can have multiple versions with different processes until you settle on what works best. I sampled the head of the source file, less than a second, to get a good snapshot. If you haven't already, sample the noisefloor, activate the de-noise plugin, and select the whole file. Then, check the box for listening to the removed sound only, and that will help a lot with what's being removed... -10 dB is a 166% reduction in noisefloor. Your 40% reduction at -20 dB is about 133% or an 8 dB overall noise reduction.
It's been my experience with this process in Audition that setting the Noise Reduction slider anywhere from 30 to 90% creates more artifacting that setting it to 100% and using very little level reduction. Drawing a curve in the freq. box really helps to only process those areas that require it, further reducing the artifacting of the process.
I hope this is helpful in any way. Must best advice would be to experiment with multiple procedures on 3 minutes of program to find the best workflow for the entire project. Once found, replicate with the entire project.
Cheers,
-CS
FYI - what I had to sample sounded like a fade-in, so I had no idea what the actual level of program are