I would send DV right away. Do you have any clue what this debt could be regarding?
Normally, you want to check SOL before DV'ing, because you don't want to wake a sleeping dog, however it seems they are already awake.
Before they can garnish your wages or bank accounts they must win a judgment against you.(I am pretty sure)
If you DV them, they MUST cease all collection activities until they validate. Filing suit is a collection activity.