How can I select the Echo Device that has the Single Device Amazon Music Plan?

TL;DR: there's no way to do it on your own. Contact Amazon customer service and they'll politely do it for you.

The Single Device Amazon Music plan is a great deal: all the music you want on a single Echo device for just $3.99 a month.

I've had many Echo devices over the years. I signed up for the single device plan way back when, when I only had one. Recently I got an Echo Studio, which has excellent audio quality and naturally, I wanted it to be the one device with access to the music plan.

The problem: when I asked for music, Alexa complained that the album in question was available via Amazon Music Unlimited which was already in use by another device.

But which one?! Turns out there's no way to find out anywhere. Not in the Alexa app, not in Amazon's full fledged website. There are a few tantalizing misdirections regarding preferred speakers which do nothing.

In the end I gave up and contacted Customer Service via email. I explained that I wanted the plan to be assigned to the Echo Studio in the Kitchen and that I could not see a way to make this change. They replied within a few minutes, politely explaining that yes, there's no way to do this on your own and they had already made the change from their side.

To complete the process, I had to reboot the Echo Studio, and it worked.

I was not able to find information about this anywhere on the net, hence this note. Hope it helps!