My understanding is that RC network adds impedance to make speakers an easier load to drive.
My guess is that typically it adds more in the case where a tube amp is used. Tube amps tend to not like lower impedances especially in the lower octaves where commonly found with many modern smaller speakers that also have good bass extension, like Merlin and many others. High power higher current SS amps, like BC Class D or many others, will be less challenged, so I would expect less benefit with those, but I suppose its always best to compare with and without when possible and be the judge.
My guess is that typically it adds more in the case where a tube amp is used. Tube amps tend to not like lower impedances especially in the lower octaves where commonly found with many modern smaller speakers that also have good bass extension, like Merlin and many others. High power higher current SS amps, like BC Class D or many others, will be less challenged, so I would expect less benefit with those, but I suppose its always best to compare with and without when possible and be the judge.