Appears the correct code to edit lives here:
products_quantity_discounts.php

I have managed to replace the dashes with pluses (- becomes +). And I think I have found the line that needs to be fixed:
$quantityDiscounts[$columnCount]['show_qty'] .= '+' . number_format($products_discounts_query->fields['discount_qty']-1);

When I try to remove the second part like this:
$quantityDiscounts[$columnCount]['show_qty'] .= '+');

Everything just goes screwy. I wonder how to change???