For Sub Command, the usually recommended mod is SCU/SCX that can be found there :
http://www.subguru.com/downloads.html
But notice that the missions and campaigns from unmodded Sub Command will be rather broken with SCU/SCX, as Sonalyst missions were not designed not for the specific SCU/SCX detection , range, etc, that can lead into problems.
By example in the "super escort" mission in which if played with SCU/SCX will have the tanker destroyed 1 minute after mission start by surface missiles shot by some Kilo from the other side of the map, and so leading you to fail the mission (as you are supposed to hunt and sink 2 Kilo before the tanker arrive on their location).
If you install SCU/SCX, to spare you frustrating time, it is better to then download and play with missions designed for SCU/SCX.
You can find a bunch of them here :
http://www.subguru.com/missions.htm