This is indeed how it should be done for adjusting the sweet spot, less is more in this case. If the listening room doesn’t suffer from really bad acoustic problems which can only be corrected by changing speaker/sweet spot positions and using PEQs, simple filtering like the Fixer offers can give very good results.
In most cases it is more natural sounding if we lower the opposite frequency band in order to boost some frequencies on the other side plus increasing the listening volume to make up for it. For example in order to lower the upper to mid bass (about 60Hz-250Hz)while increasing the volume a bit, which will be a natural reaction of our listening experience, will give more dominant high-mids/treble (compared to the bass) and - maybe - a wonderful blooming midrange (between 300Hz-2k) with some new and pleasing sound elements in some cases. Very interesting and new combinations might be experienced keeping this mechanism in mind.
If we would simply increase the high-mids and treble – which seems at first of course more logical if we miss this range and want to boost it – we will achieve rather the opposite results in many cases and cover the low-mids (250Hz-2k) and make the bass appear out of balance compared to the high-mids/treble the more the louder we listen. The reason is that bass and mids/treble don’t react with the room acoustics in a linear way if we increase the volume and if the room is not acoustically treated.
There are of course no final and fix rules in this game but as a general start I would always reduce something before I would try boosting it. The midrange is a very sensitive area and rather difficult to adjust without PEQs and some background knowledge. Our experience of "timbre" and "color" of sound events comes from this very important frequency range. Lowering the bass can do miracles as we don’t only affect the bass frequencies which we want to tackle but also their harmonics which can appear still very audible in the upper bass/lower midrange. Often the first harmonic is more present than the fundamental. If we think we hear a 40Hz low bass of a bass guitar for example we normally hear in our room rather the 80Hz harmonic which is considered as in the range of mid bass where we start to perceive "tonality". With a problem in the room acoustics it could then appear too loud also at 160Hz in the upper bass range.
The Fixer with its corner points of 300Hz and 2k starts to work at the low end of the low-mids and the beginning of the high-mids which seems to be a good compromise for not offering specific mid-range control.