Adrien, in the past I briefly tried a sub located near the listening position and quickly put it back into the corner. Besides the bass frequencies themselves, moving the sub closer to you may make other sound more audible and localizable, such as mechanical vibration of the enclosure and port noise. Still another effect that I experienced was the psychological one of knowing that the sub was so close and not located near the mains.
On the question of harmonics, they can be both "natural", creating the characteristic differences in the sounds of instruments playing the same primary frequency, and harmonics resulting from distortion in the sub. The rolloff(typically at 24dB/octave)which the receiver crossover does is generally sufficient to keep a 80Hz crossover from being localizable due to higher harmonics, although of course there's no sharp dividing line at that particular frequency. Harmonics created by the sub itself as a result of distortion aren't reduced of course by a crossover in the signal feeding the sub, but it seems unlikely that a sub of the STF-2's quality would have high levels of distortion. My view would be to restore the original settings and location, but contrary to usual practice you can try activating the internal sub crossover. The STF-2 has a nominal 90Hz highest crossover frequency(Nousaine's tests have shown that these control markings are inaccurate on many subs), so you can set it there in an attempt to cascade the receiver and sub crossovers so as to roll off the sub output above 80-90Hz more rapidly.
