If a particular dip in your FR doesn't respond to EQ attempts, it's most probably a room induce null. Nulls tend to be sharp and deep. Some people locate their sub where there are fewer nulls and then only cut the peaks... There is nothing wrong with boosting (IN MODERATION) when you are sure it's not a null. You can't overcome severe nulls with EQ, period. Just watch out for cone overdrive, loss of headroom, and amp clipping...