Extra bass does boost the bass, it gives you a peak in the frequency response centred on the crossover frequency, so it's not really a good thing.
Remember that there is not a sudden drop in a speaker's output at the crossover, it drops off gradually (same for the sub above the crossover). When you turn on Extra Bass it has no effect on high frequencies above where the sub plays, and it has no effect on the lowest frequencies where the main speakers have no output. But there is a range near the crossover where both are producing some output and in this range the combined output is increased, which ruins what otherwise could have been a flat frequency response.
If you want more bass it's usually better to have the frequency response gradually rising at lower frequencies, rather than have a hump around the crossover.