homepod 1st Gen has lost all Bass sound, inexplicably. Is there a way to do a speaker diagnostic?

The OG HomePod is long out of warranty and the cost of repair or replacement is about the same you would pay for a new one. The only thing you can try which is not guaranteed to work is to try these steps:

• Unplug HomePod for over 24 hrs
• Plug back in - Wait at least 10 seconds

1. Unplug HomePod or unplug the power adapter for HomePod mini, wait exactly 10 seconds, then plug HomePod back in.
2. Wait exactly 10 seconds, then touch and hold your finger to the top of HomePod.
3. The white spinning light will turn red. Keep your finger down. 
4. Siri will say that your HomePod is about to reset. When you hear three beeps, you can lift your finger.

After you reset HomePod, you can set up HomePod again.

Reset HomePod or HomePod mini - Apple Support

Quite literally just happened to me as well, makes a pop sound and woofer seems to turn off?? I have to disconnect and reconnect and it's back but it keeps popping and turning off after a bit. Truly unfortunate because the bass was the only reason I got a 1st gen HomePod...

Hello!

Contact Apple Support, and they'll be able to tell you if they can run some diagnostics for you: Contact - Official Apple Support.