Hi Rob
I wouldn't know! I have never seen those elusive HOMs myself.
The M2 waveguide is surely packed with those, so much so that they stop being a problem in the end
Funny to think that Earl Geddes took two opposed routes for dealing with HF and LF problems: trying to suppress any defect for the HF (and ending up with a big one on axis, that he deemed irrelevant in his preferred listening scenario) while trying to mix and compensate different modes together with his multi sub approach in LF.
The M2/sub18 system is the exact opposite
I guess both approaches can work, as well as many others probably, as long as the system is coherent and well thought out.
I think the same goes for measurements: you can look at them in many different ways and representations, but this is still the same info. No need to fight over this.