If anyone else comes looking for this solution, here was mine: i checked (enabled) "Download newly available updates in the background" in the App Store System Preferences (not install!), waited a while (noticed softwareupdated was present and active in Activity Monitor), then toggled Siri on and off, swapped to another language and then back to American English. After this fiddling (the same on two machines with the poor quality voice), i now have the correct high-quality Siri voice.
It seems Apple expects automatic updates to be enabled and has tied system component downloads to this function.
EDIT: i found this solution on some other site where people were complaining about the same problem. i didn't take note of the site and cannot find it in my extensive internet history, or i'd point to it here.