Looking things up, I find two unit abilities: ABILITY_RANGED_ATTACK_DISTRICT_DEBUFF and ABILITY_BOMBARD_ATTACK_UNIT_DEBUFF. They have descriptions LOC_ABILITY_RANGED_ATTACK_DISTRICT_DEBUFF_DESCRIPTION and LOC_ABILITY_BOMBARD_ATTACK_UNIT_DEBUFF_DESCRIPTION that mention the -17 (defined in Unit_Abilities_Text.xml).

The abilities themselves, however, are text-only. No associated modifiers. The actual combat effects are elsewhere. So that part only helps you with the tooltip.

And that leads us to GlobalParameters.xml. A few key lines excerpted:
<GameInfo>
<GlobalParameters>
...
<Replace Name="COMBAT_BOMBARD_VS_UNIT_STRENGTH_MODIFIER" Value="17" />
...
<Replace Name="COMBAT_RANGED_VS_DISTRICT_STRENGTH_MODIFIER" Value="17" />
...
</GlobalParameters>
</GameInfo>

Updating that table might actually do the trick.

Thank you! I'll try.