You adjust ranges in Library
SH4/Data/Library/USSubParts/SensorData
You'll find 3 visual nodes inside in a row, seems it starts around node 29. Just increase ranges
Also inside the Library itself is another sensorvisualsub.sim where you can adjust visuals.
Then you can tweak with the sh4/data/cfg/sensorscfg to fine tune the above.
The trick is finding realistic ranges that work with the env. mod you use. They're several factors in scene dat and env files that effect visuals for sub and AI. Some env mods have really dark nights with dark horizons and often the enemy sees you before you see him, can even happen during the day that the enemy shoots at you before your crew sees him. I use my own enviroment mod and use a lot of moonlight, so horizon with the moon you can see to the horizon, away from the moon it's fairly dark. IMO your sub crew should spot the enemy before he spots you.