Dice Roll Modifiers
Some effects modify a specific die roll, such as Sundering (Defenders suffer -1 to Defense Save rolls.) It is important to note that these effects modify the result of the roll but not the action die-facing itself. This means that if you had another effect that specifically triggered on rolls of a 6, it refers to the actual die-facing and not the roll after modifiers.
Additionally, effects can never reduce a die-roll below 0 or an individual die-result above 6.
A unit with a Morale Stat of 6+ is forced to make a Panic Test and is currently suffering a -4 modifier. They roll a 1 and a 2 on their dice. Their result would be 0, as their final total cannot be reduced below that. Unfortunately this means the unit is going to lose 6 models from this Panic Test (since they failed by 6). Ouch!