Just because firmware 5.0.617255 is generally available doesn't mean, everyone who's
not on the beta programme already has it.
Remember:
forum.wiimhome.com
If you don't have measuring equipment the simple rule of thumb is: Whatever setting gives the highest SPL (read: loudness) around the crossover frequency is the right setting. Chances are, 0 ms (no delay at all) might be the best setting anyway.
If your sub is performing some digital signal processing on its own then it most likely imposes some delay. The manufacturer of the sub should be able to specify this number in ms. There is no way to reduce this delay. Period. Choosing in the WiiM Home app will actually add a delay to the main speakers output to compensate for the delay of the sub. Negative numbers for subwoofer delay will actually add a delay to the mains output.
Only if your subwoofer does not impose a (noticeable) delay
and it is placed further away from your main listening position than your main speakers you will have to go with a positive number in the delay settings for the sub.