Update: I managed to snag an AS-P4100 subwoofer here in Toronto (funny enough, even though the Athena's were made here, it's easier to find an online retailer based in the US......). I hooked it up with my WS speakers and now it sounds great! I find that it blends better with the Athenas than the Energy S8.3 that I was trying out initially.
Here's the new measurements after re-running Audyssey - 70Hz for L/R, 150 for the center, 100 Hz for L/R surrounds, 100 Hz for the LFE. I had manually set the crossover control on the subwoofer to 85 Hz (roughly where the WS-60 setting would have been marked on the previous AS-P4000).
Overall, I am happy with the sound, I think I just have to tweak the treble settings a bit to make it sound a bit "brighter". Any comments on the crossover settings, please let me know if I can still improve on these. I also hope this helps out other folks with the WS series.