Your bias is understandable, but this conflicts with a lot of the experience of recording and mastering engineers who find precisely the opposite with regards 'bloopy bass':
...and—sorbothane has "reverberation and resonance issues"??
Hi, Russel. Square Fat Dots being a superior isolation material for monitor speakers is not a matter of conjecture or bias, but proven in real-life application by hundreds if not thousands of customers who have upgraded from Sorbothane footers to Square Fat Dots under their monitor speakers. A "bloopy bass" is just one example of the sonic anomalies caused by highly vulcanized polymers in audio applications, and a "bloopy bass" more so applies to rubber footers as opposed to Sorbothane footers (depending on the particular vibrational environment each is dealing with). Sorbothane is perhaps mid-fi at best, being somewhat effective as an isolation material but virtually always at some cost, such as decreased linearity, attenuation or accentuation of higher frequencies, or other anomalies.
Rubber, Sorbothane, Vibrapods, and other highly-vulcanized polymers act like springs. Like springs, they rely on a very narrow range of weight load in order for their contra-vibrational energies to be effective. The range of vibrations that are diminished is also limited while at the same time some reverberations will occur due to their tightly cross-linked cellular structures. This results in inefficient vibration-absorption and loss of sonic linear integrity. Molecular and cellular structures of vulcanized rubber and synthetic rubbers are tightly cross-linked for industrial strength, not for audio isolation purposes, which makes them poor materials for vibration control in the higher-frequency, acute micro-vibrational realm that you need to deal with in the audio and video environment. Most rubber-like materials are too "slow" with their contraction/decompression properties to have any effect on very acute high-frequency vibrations that cause a lot of the grunge and glare in the music.
What difference does the shape of the 'dots' make? How many will I need under each speaker? Will they definitely control the distortion/resonance? I presume I can return them if not?
Is it necessary to isolate the slate plinths?
George: The shape of the Dots has little to no effect: Square Fat Dots fit nicely into the corners of the top plate of many speaker stands, while Big Fat Dots have more isolation and decoupling "beef" due to their larger size. Four under each speaker is usually sufficient, with perhaps an additional one in the center being additionally beneficial. dBNeutralizer material has little to no inherent resonant issues due to its lightly vulcanized and loosely cross-linked structure, though quite firm and non-squishy--providing efficient isolation and vibration damping without introducing any distortions or resonances.
Isolating the plinths isn't usually a necessity but would likely be beneficial. Isolating the speaker itself is the most essential area to address, and isolating the stand and plinth adds extra layers of isolation for the speakers. Addressing the stand-to-floor interface is always beneficial; Gliders provide superior vibration control as well as easy mobility when needed.
Herbie's Audio Lab offers a 90-day return policy on all of our products: we don't believe you should be stuck with something that doesn't work for you, and sometimes it takes some time to get everything set up optimally. Whenever a new variable is introduced to the system, including adding isolation materials to components, some adjustments are sometimes needed for everything to fall into place, whether it be a bit of break-in time, fine-tuning of speaker positioning after phase relations are corrected at the speakers, or other adjustments of the system.