Why Bass EQ
When movies are mixed in the studio, the sound will be adjusted according to what they hear in that studio. Sometimes limitations in the playback equipment, such as speaker system, will lead to adjustments that will compromise sound quality. Typical examples of such undesired adjustments are high-pass or shelve filtering that removes the lowest frequency content. On a limited system this may sound better, because the removal of content that can not be reproduced anyway will increase headroom for the mid and upper bass frequencies. When played back on a full frequency range system the filtered version looses weight and impact, and also tend to sound less natural. If this filtering is done in one of the very final stages of production, then it is a good possibility that at least some of the filtered content can be retrieved. This is what Bass EQ tries to do. The better your playback system is, and if you also like it loud, like 0dB/reference, then the difference will be very significant, and on some movies the whole experience is lifted to another level. But also if you play at lower volumes, say -20dB, perhaps you have some smaller but still nice and good quality subwoofers, then the difference will most certainly be very noticeable. This is not only about more shake and physical impact at house-wrecking volumes, the overall sound quality is improved when the natural wide frequency range is restored. Example frequency spectrum plots playing this scene from the movie Battleship:
Battleship, original and Battleship with Bass EQ playback:
There is a huge difference in output at low frequencies, and listening easily reveals that the perceived increase in sound quality is significant and very far from subtle.