A temperature of -50C is below -40C. Likewise for dB; -50dB is below, or less than, -40dB.
For power, -40dB is a ratio of \$\frac{1}{10,000}\$ while -50dB is a ratio of \$\frac{1}{100,000}\$.
The only general rule of thumb I can think of is to balance loading whenever possible.
In a three phase system, when harmonics are generated at any point, every mitigating measure is some variation of getting them to dissipate as harmlessly as possibly, through or across some kind of reactance. One way of getting them to dissipate is by simply allowing them to flow through devices that are closer to the non linear load than the service entrance. The load creating a harmonic can only supply it so much power, and by sinking that through another load, it doesn't make it's way back to the mains as well. Some harmonics negate entirely when balanced. Occasionally facilities that get in trouble for harmonic generation or effective power factor can get back within limits simply by balancing their phase loading.
Of course, if this is the strategy, then the linear loads near harmonic generators will have to be rated to handle the extra heat generated.
And it seems like a higher per unit impedance would attenuate harmonics leaving or entering a facility; so generally speaking, I suppose it is safe to say that a higher short circuit capacity would allow higher amplitude harmonics simply by not stopping them.
As far as your gigantic light bulb theory, we can reason it out. If they draw the same amount of instantaneous power, then they draw the same amount of instantaneous current, which suggests they have the same impedance at the fundamental frequency. Of course, that impedance will be higher for higher order harmonics, making the motor less effective at harmonic sinking than the resistive light bulb. In reality, though, that light bulb is enormous, AND you need three of them - and you probably don't need that much light. But, maybe this makes a case for placing lighting loads alongside noisy loads?
I suppose the next question would have to be, can a halogen luminaire bulb handle the harmonics?
Best Answer
Weighted THD is preferred over THD when an inverter is generating non-sinus waveforms. Take a BLDC machine, this is usually excited with a trapezoidal current waveform
THD aims to express the quality of a sinus so if what is being generated isn't a sinus its benefit of determining how pure the output waveform is (w.r.t. what it is expected to be) has been diminished.
WTHD aims to to predict the distortion in the current (and thus the additional losses)