Thanks for the feedback Trebor - I really do appreciate it (much better than the deafening roar of silence as tumble-weed rolls across the forum page 
)
The example you have posted has much more severe damage than this plug-in is designed to deal with. In truth the only “effect” I would consider using on such a low quality sample would be the delete key (unless of course it was an irreplaceable once-in-a-lifetime recording with personal value). In my description I use the phrase “moderate amounts of hum”.
Here is an example at the high end of what I would describe as a “moderate amount”
(apologies to Koz for butchering his lovely piano sample - I don’t have a suitably buzzy recording so I had to record some buzz then mix some audio with it. The buzz is a recording of my finger on the end of a jack plug straight into the sound card).
Here’s the results of the two hum removal effects:
The first section is using your de-hummer and the second section using my de-hummer (not intending to suggest one is “better” than the other - just a matter of differentiating between the two effect).
The first de-hummer certainly removed more noise than the second, but the damage done to the piano is clearly audible.
Of course there is no reason why these two effects could not be combined. The “new” part of this de-hummer is that it makes a smooth transition from filtered low level sound (where it is most needed) to unfiltered high level sound (where it is not needed). There is a degree of “distortion” that occurs during the transition due to difference in phase, but it is pretty negligible and a good trade-off.
Just looking at your effect again - it’s pretty similar to the notch loop used in this one. The main difference is the algorithm for calculating Q.
I’ve deliberately left out any settings for this for the sake of a simple user interface and instead opted for optimised* settings for low to moderate amounts of hum.
(* only roughly optimised - a lot more real world samples would need to be analysed)
The other difference is that you have used a lot more iterations. The number of iterations for “even harmonics” in my plug-in are artificially limited by the slider range but can be overridden through text input right up to half the sample rate.
As well as the damage caused by removing significant amounts of frequencies (the dull sound), there is also the problem that you mention in your previous topic of steep notch filters causing ringing. This will be particularly noticeable with transients, but are virtually eliminated with this “new” plug-in because transients are not filtered.
The other difference is in separating out the odd and even harmonic into 2 loops as per Koz’s comment https://forum.audacityteam.org/t/new-plugin-super-bass-cut/13382/1 . Doing this makes a significant reduction to the amount of damage when dealing with lower levels of hum. My own interest is in looking for ways to reduce low level hum for cleaning up otherwise high quality recordings, so my priority is more concerned with minimising the damage and I’m looking for ways to reduce the damage further.
[Edit] I see there’s been further discussion while I’ve been typing - I’d better post this quick