I'm not really familiar with Modern ASW Warfare.. all my subsim experience is in either a Gato, Type 9 or Type VII.
Because of this, the difference between Narrow Band, High Frequence, Low Frequency, Medium Frequency Sonars is zero for me. I have not a clue what they're individually used for, so bear that in mind when I comment
As you say your contact is an SSK, thus meaning he's a diesel electric submarine, could you not try to keep within the general vicinity of the submarine and wait until he snorkels to recharge his batteries? I'm not sure if there's a specific frequency of detecting those sounds at long range, but I presume a snorkelling submarine is pretty loud, not to mention susceptible to being detected by radar (Does ACTUV have radar?)