Use the command line with unXWB with the -R option, this will give you the raw XMA without the added header.
Then use the XMA converter with VGMToolbox (as long as you have ToWAV in the required folder for VGM Toolbox to utilise it)
Set the Frequency to "48000" and the Channels to "Mono (1)" and have the Execute ToWAV.exe on the output files option selected.
A few in RD21 need to have the proper header added. You can use QuickBMS from Aluigi and use Alpha23's AddXMAHeader script on the .DAT files that VGMToolbox doesn't convert.
Or you could just run all of the extracted DAT files through QuickBMS with the script and then run the XMAs through ToWAV.