What happens if I string an Ethernet cable between my stereo-paired PLAY:1s?

The difference from 70/69 to 63/67 could simply be accounted for by the change in local conditions: the presence of the wire, the fact that the PLAY:1s Ethernet controllers are active, etc. etc.
Yes, but why would the two remain in secondary mode with mid thirties yellow boxes with a 70/69 wireless signal between the two, but start using the 63/67 strength wireless connection by dropping to tertiary mode as soon as a wire is introduced? Introducing the wire gets one to prefer to wirelessly connect to the other, while the absence of the wire leaves both in direct communication with the Boost. So the wire appears to be making a difference in their behaviour without itself being used for signal transfer.

Something doesn't add up. With the wire in place -- and a 'tertiary' label -- the wireless path should be blocked. You should see no coloured squares in the relevant matrix cells.

STP will strive to minimise the aggregate 'cost' of the path back to the root bridge.

When your two units were unwired, signal strengths to the BOOST in the mid 30s were perfectly viable, so each unit made a wireless connection thereto.

When you wire two units together the 'cost' of the Ethernet link is 10. This means that the wireless STP root path costs of the paired units -- which are a function of signal conditions -- have to differ by at least 10 for the wire to be 'cheaper' for one of the units than to use its direct wireless path. Assuming that's the case, the wire will come into use and said unit will become 'tertiary'. Its root path cost will become that of its twin plus 10.

With the wire in place -- and a 'tertiary' label -- the wireless path should be blocked. You should see no coloured squares in the relevant matrix cells.

Although I hate the phrase, all I can think of is: My bad.

Going back and forth between the two, I was confused and erred in saying that the wired connection has a coloured square; I added a wire again as soon as I read the above, and you are right - no coloured squares.

So, the tertiary mode is working via use of the wire, and one cannot see the signal strength it delivers, in the matrix.

But since both units work fine without it, the wire goes back into storage to reduce clutter.

So, the tertiary mode is working via use of the wire, and one cannot see the signal strength it delivers, in the matrix.
'Signal strength'? It's wired!!! It will be delivering 100Mbps.

Yes, but the question was in the context of what that is if it was used as a marker for how good the signal strength is if the matrix says it is 70 as an example, for a wireless signal. Is that 70% as good as that
from a wired connection? Or am I doing apples v oranges?

So, the tertiary mode is working via use of the wire, and one cannot see the signal strength it delivers, in the matrix.
'Signal strength'? It's wired!!! It will be delivering 100Mbps.

In my case I'm using very expensive audiophile-grade Cat 5e cable. The bits will be better, faster and more plentfiul than normal cable.
🙂

Hah...but I am treating the air in my home!

Or am I doing apples v oranges?
Definitely.

Bandwidth and latency might be common external parameters with which to compare wired and wireless media. But the signal strength figure is just one small part of the equation. How wide is the channel? What encoding scheme does it use? What's its error rate due to interference? Etc. Etc.

A case sort of in point: As I noted in an earlier post, a PLAY:1 literally underneath a BOOST (signal 96) ostensibly couldn't 'hear' it because it was being 'deafened' by it.