If we are talking about the bass end, then I would nominate the correction built into the Velodyne DD range of subs.  They are shortly to release a stand-alone box of electronics that does the same job - SMS-1 - for a reasonable amount of money, IMHO.  Having already added some room treatment up to the level it begins to intrude domestically, I'm very tempted to try one of these things (or the sub itself).