Improve destroyer's counter to subs
Here is a concept for consideration.
Both subs and destroyers start at 3/3/2. Improved subs get +1 attack while improved destroyers get +1 defense. Advanced subs get +3 attack while advanced destroyers get +3 defense.
Another option is for subs to start at 4/3/2 with +1 attack for improved and +2 attack for advanced while destroyers start at 3/4/2 with +1 defense for improved and +2 defense for advanced.
Either way, subs and destroyers have equal attack vs defense as they tech up.
That leaves the subs ability to sink ships without them being able to return fire.
I don't think Hepps wants to remove this ability altogether, so what I propose is to add a defensive depth charge to destroyers that fire each round of combat that is strong enough to achieve parity with subs (i.e., when an equal number of subs attack an equal number of destroyers—of the same tech level—the result is a 50/50 battle).