Found the answer to my own question and it is: there is a check box under file handling in Photoshop preferences regarding 32 to 16 bit conversions. The default is to use ACR; so uncheck it.
What I am doing is a new HDR process; don't know who deserves credit for this but I learned it from Lewis Kemper. It is probably already here on this forum somewhere, but here are the steps:
Select files in Bridge
Tools/Photoshop/Merge to HDR Pro
Select 'Complete Toning in Adobe Camera Raw'
In ACR: set all options to suit as usual.
Image now 32bit
Flatten [Or keep working as smart, 32 bit file.]
Convert to 16 bit and select 'Exposure and Gamma' on dialog
[Must first set a "file handling" option in preferences to prevent this step jumping back to ACR a 2nd time]