Meridian's approach is the best I have found (both sound quality wise and theoretically) for electrically correcting the problems caused by a room. The Meridian digital room correction system follows the same high level process as other devices such as those by TacT and Audyssey (measure, compare, generate and then apply correction filters) but utilizes the reverberation time versus frequency response of the system. The system analyzes the deviation of the measured reverberation time from a target. Since the Meridian system only operates below 200Hz (i.e. below the transition frequency) it can take advantage of the minimum phase behavior of the room in this region, allowing it to correct reverberation time (or more correctly modal decay time) through magnitude based correction filters. Please read their white paper here.