Given that most people do not buy great subwoofers capable of high output, corner placement is probably best for most people, as it increases the effective output. But one should try it and compare with other placement options, as it will not always be best.
As for EQing a subwoofer, if you have the equipment for it, that can help quite a bit. But only for getting rid of peaks, not for filling in significant valleys in the frequency response, because it takes considerable power to do that, and very often, neither the woofer nor the amplifier is not going to be able to handle it. Remember, a 10dB dip would require 10 times the normal power to fill it, and that is just not going to do doable for usable volumes with normal subwoofers.