Another problem the defiant has is that 6 WB P/SB are equivalent to 4 Launch Bays, yet it trades those for just 2, making it under-armed in comparison to the Endeavour and Endurance, as well as being more expensive and eclectically armed.
Obviously it can't have 4LBs, so that leaves doing something sensible with the remaining armament. Perhaps +15cm on the lances, or 2 torps in addition.