Seems strange as there are articles on the web showing you how to set up the equaliser for the HomePods. Can only assume the latest firmware has somehow locked this function out?. No big deal I just lower the volume slightly which helps.
I don't think AirPlay 2 protocol supports EQ adjustment at this point anymore. You can set a Macs system wide audio output to a HomePod (not stereo) and adjust EQ in the the music app (AirPlay 1). Keep in mind the digital EQ in MacOS Music app isn't that great, you can cut frequencies but boosting them hurts quality. EQ presets in iOS is arguably worse because they will boost frequencies.
The HomePod is equalizing its audio based on spatial awareness and the audio mix/quality being a known variable with Apple Music. HomePods in a stereo pair are using different EQ's based on placement which doesn't leave much room for manual adjustment without harming their balance in short order. Their well balanced EQ is why they can fill a room with full sound using horned tweeters and a high excursion driver.
I do agree that user preference is more than enough of a reason to have adjustable EQ. However it doesn't seem that people want (need) an equalizer though, they just want to reduce some of the lower frequencies.
That being the case Apple should be able to implement a "reduce bass" toggle in the HomePods home app (filter). This could limit the low frequencies specifically for the HomePods, while maintaining balance between 2 and not effecting other speakers on the system (AirPlay 2).
