if you filter the AC b e f o r e the standard PWS you help the PWS with its very poor EMI/RFI suppression (if any) BUT
the awful negative effects of the Switching PWS itself happily enter the SB3 unhindered.
So a filter a f t e r the switching PWS will be VERY beneficial.
A very good and affordable filter in the 5V line of the (standard) switching PWS is the Felix
but then you do not protect the rest of the system from the SW PWS
SO try with two Felixes on in front, one behind the PWS