Coop Board President in NYC (Queens) for 14 years. Safe to say, all manner of noise complaints have been brought to me for resolution.
Granted, so much depends on the building structure itself. Prewar, wooden floor joist buildings are going to transmit all manner of sounds and vibrations. Sheetrock walls, unlike plaster walls, do not muffle sound.
What can you do? Point #1 - Nothing and I repeat nothing (speakers, electronics, speakers, TVs) should share a common wall with a neighbor. All interior to you.
In your case, you directly coupled your speakers to the floor. Simple solution ... de-couple them. I would not be very open to whatever arguments regarding your music that you would make. You attached your speakers to a common surface.
Point #2 - 8AM is too early to start in with your music. Just is. I am writing this at 9AM on a Tuesday and I would be super PO'd if music were lofting in from above me.
In my building, we have seen the proliferation of pianos and children practicing. Parents get their kids started early on weekend mornings. It's awful.
Add to this mix ... municipal construction noise, lawn mowing, cars, buses ... you get the idea.
You did mention you have good soundproofing in your place, Could be your neighbor just got tired of the morning symphonies? You would think if everyone survived WFH, you would be ok going forward. Maybe they just retired or had a change in work set-up. It had to be loud enough to pass through 2 rooms and elicit a complaint.
Still ... you are in a cooperative living situation ... cooperate.
I would love to go loud at times with my stereo ... I just don't. If you live with someone, what do they think of the sound level?
Rich