New development...
After spending 30 min on the phone with Apple support yesterday, the Sr. tech I was speaking with, was stumped.
All the settings and statuses we checked were acceptable, and not the source of the problem.
So, my theory was...something in the MacOS 10.13.6 update broke my eq.
The tech and I decided to try restoring my Mac to a Time Machine backup I had done just before updating.
I did this last night, and viola!
I'm back to using MacOS 10.13.5 and iTunes 12.8.0.150 and all is well.
I can now send Apple Music simultaneously to my HomePod and Mac speakers with a functioning equalizer.
It really makes a nice stereo effect without having to have 2 home pods. 😉
The tech said he will call me back next week to check status.
If my experiment was successful, he would send the issue up the chain of command so the developers can de-bug their MacOS and iTunes update code.
The take-away from all this is...
If you like using iTunes eq with your HomePod, either make a time machine backup before updating, or skip the MacOS 10.13.6 update.
Also keep in mind...
Everyone's MacBook is unique in configuration, what and how apps are installed, and how everything is affected by updates.
Much like how medications affect different people differently, the MacOS update that disabled my eq may not affect your Mac in the same way.
So, the old rule of thumb is reinforced here...
Backup before updating.