Use a Seawolf.
If you don't, well, you're going to have to compromise eventually. In a sub v. sub fight, unless you are in a 'wolf and he's in a Han screaming along at 25 knots, you pretty much have to assume that he sees you and is trying to get a solution on you. And the first to get a solution and fire is usually the first to win. A winning fight is one where you can walk (swim?) away and he can't, 'cause he's in a thousand pieces scattered across the seafloor.
Get up to tactical speed, put him on a lag LOS, and keep making new TMA legs to keep him on his toes while you get a solution. Once you've got that solution, fire at the best tactical moment (very situation-specific), which may be as soon as humanly possible or twenty minutes down the road when you've slinked in behind him. Keep in mind, though, that if the bad guy is also experienced, he's going to use all the tricks in the book to get to you first and make your TMA life a living hell in the process.
I am interested, though, in how you know what you're shooting at. By the time you are able to reliably classify your target, he should be close/loud enough for a BB tracker, and thus DEMON. I mean, that 50Hz-125Hz contact on your NB could be a fishing boat or a Kirov, and everything in between...