Unfortunately the Play1s do not have a line-in and the Echo Show is the one Echo in the current product mix that does not have a line-out. Some options are:
Try the new version of the Echo Show to see if the improved sound will suffice for your needs. It is supposed to be much better than the previous version.
Get a Connect plus a Bluetooth adapter and use the Bluetooth out of the Show to the Bluetooth adapter connected to the Connect to play through the Play1s (not recommended)
Get a Spot instead of a Show, plus a Connect and use the Line-out on the Spot to feed the input of the Connect to play through the Play1s (also not recommended)
Maybe others will think of additional ways to approach this.
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