To put it very simple. If we for the sake of argument, forget that speaker drivers can have more than just linear distortion. Then a speaker driver can only move in and out - 2 way - 2 dimensions.
This means that an equalizer - both analog or digital - like the miniDSP, has the potential to adjust the level of this - in and out motion - at all frequencies, until you get an equal amount of energy from - let's say - a mid/woofer like the SB17 driver. Of course there are limits, the driver will never be able to play deeper or higher in frequency, then what it is designed to do, but within it's workable pass band, we can smooth it out and get a better and more neutral response.
Then follows two problems.
Firstly, there are limits to how much and with which filters you adjust the levels with any DSP or equalizer. EQ simply moves the phase too, which means that you move some frequencies in time - more than others. Luckily within certain limits, we simply can't hear this, but it all adds up, so again we have to work smart with these tools.
Secondly, as soon as the sound leaves a speaker driver - all comes into play. Flush mounting the driver, edges on the cabinet, the shape of the cabinet, all surfaces around the speaker and the distance to each and every one of them too.
This is very important. Because everything that comes out of a speaker, plays in many directions - but the speaker driver can still only move in and out.
All the soundwaves that emit from a speaker, will at some point hit a wall, floor or something else. As a wave expands through the air, it will hit many objects and each time reflect in new different directions and at new smaller levels. But in the end - everything gets mixed up. So if you measure the speaker with the room in one exact place in the room - you will get one exact and unique response, because of all the countless reflections of that specific room, with that specific placement of your speaker and that specific placement of the microphone.
This means that the original change that you made in the DSP/EQ, can only be true/correct - at that one spot in your room - not even the same for each of your ears - even if you sit at the exact same spot every time.
Also timing comes into play, because we as humans, has a limit to how easy we can separate sound in time - which also change with frequency. This is why dampening our rooms a bit, really helps with sound quality, from any speaker. Very roughly speaking - any sound reflection that arrives to our ears before around 20ms - smears the sound. And if they are delayed more than around 20ms - it adds "room" or spaciousness. Again - this a rule of thumb. It is a bit more complicated when going into detail
This is why we measure off-axis. Because if we can build a speaker, so that it sounds the same in all directions, then all the reflections will be much closer to the original sound and therefore not ruin the sound image - even if we move our head around during a heated moment of our favorite music.
To make it the most precise and neutral sounding speakers, that does not intervene or adds its own "sound flavor" to the music. We simply have to ask ourselfs, how far we will go and at what level something might annoy us. Some can live with a bit more distortion from a speaker, and still be very happy. So everyone does not need to dig for perfection, but might just be happy to smooth out the worst problems.
You can't correct the room, because of what I just wrote above. Many people average what they measure and stay happy with what they can get, with a bit of smoothing of the worst problems - simply ignoring the rest. And for sure, this can improve things quite a bit.
But if a speaker has a design flaw, that creates an uneven dispersion, a suck-out between drivers, an edge resonance or maybe a too narrow cabinet, that will not support the lower frequencies of a midrange(baffle step) - then you can't correct this with EQ. Because all these challenges, are all in 3 dimensions and complex by nature - and both the speaker driver and any manipulation with any type of EQ - can only change the "in and out" movement - which equates to 2 dimensions.
FIR can correct the phase, but then delay's all frequencies in time, to fit the lowest corrected frequency - and it is not simple to get it right, without sounding really weird. Again - a change should have more pro's than con's, to be worth it.
To sum up. A DSP is a wonderful tool. And especially at low frequencies, we can really do some wonders. But higher up in frequencies - things start to change, and we can no longer use the same tricks.
2D problems can be solved with EQ - a 2D tool. Room problems are in 3D - and needs to be corrected in the room.
You can do very cool things with a DSP - if you have more drivers and amplifiers, like with Beolab 50 and 90, or Kii3. But that is a bit above DIY in many cases.
Hope this helped in some way. It's not an easy issue to wrap our head around. So sometimes we need to read about it in different ways, to finally grasp the ways that sound and our hearing works. I still read about it, to better figure out, if I understood at all, what I was doing - cause it's a BIG subject
