Passive torpedoes are better against loud contacts. AI uses both passive and active CM. So there is not much reason for doing so. Some humans don't use both CM so this can help in MP games.
Maximum operation speed should be 5kts. Every knot makes you louder. Only go faster when you know the situation, and even then, do stops time to time.
For sure use towed array and narrow band screen for first contact with anything. It has the most range. There is also problem that weak NB (narrow band) lines are darker then the screen and they are really hard to see, especially on some monitors.
Also be sure you don't drag TA (towed array) on the bottom (you'll see bright noise on the screen). Don't extend your TA to the full, 1/3 is enough.
Go to
www.subguru.com and download guides and tutorials, as well as some cool missions.
As for your mission. Collins is really hard to beat. Forget it without TA.
Also some quick missions are impossible to complete. They are just random. Sometimes the target is too far away, sometimes it is too close (I once collided with enemy Kilo in FFG just few seconds after mission start).
Btw. you can store random seed of the quick mission (it is also written in player's log). Post it together with other info (mission type, location etc) and we can try too.
Also .. what you mean by difficulty ? There is no such thing in DW.
Edit: Oh I see .. you mean quick mission difficulty. That is just mission setup. Number of targets in general. Against sub, Easy means one enemy sub, one allied surface ship (which often sends target location via link). Medium means one target,no allies. Hard means 3 or even more enemy subs, no allies.