Nulls will generally occur in specific room areas. As Kal mentioned; you might want to try moving your listening position closer to, or further from your speakers. Often, it only takes a very few inches to make a difference.
how do you trear rooms to bring up the null points
I understand absorbing or equalizing out low frequency standing waves that are reinforced by room dimensions; but how do you bring up the sound at frequencies being attenuated or canceled out? Are there decent looking panels designed to emphasize and reflect particular frequencies? Isn't this potentially more affected by the specific listening position? This would seem like a much tougher problem than absorbing specific frequencies.