40Hz is about -15dB anechoic, that’s a lot of room gain needed.
Before correction, in my room I get about 15db boost at 38-48hz as long as the speaker has enough output to excite the mode. Nearly all of my speakers can excite this mode.
... I guess with 1st order electrical filters they have 2nd order acoustic slopes. But here I'm unsure if it can be blamed also on the broader overlap or just some error in crossover symmetry.
Applying a 1st order electrical filter to a driver can give you all sorts of various acoustic roll-offs. There is no reason to expect a 2nd order roll. With excellent design and drivers often a 1st order will give acoustic rolls of 2nd and 3rth order (even 4th). You can also get no roll off at all in case of a hypothetical driver with a severe break-up & or severe rising response.
I have to say that I don't own any back ported speakers now for this very reason. I am surprised that I read that 5 cm Genelec recommendation pop up quite often in the comments, and more generally speaking often being mentioned that rear ports distance are not really an issue as long as their is a bit of room to breathe, often from quite knowledgeable people. I am missing technical knowledge about that, but I did have rear ported speakers in the past and I could swear I was hearing negative effects at distances of about a feet and that the effect is just much less present with front ports, but having not compare or evaluated lately, It makes me doubt my ears, or maybe It could even be about something else than the port and I was not comparing apple to apple, or maybe the good old expectation Bias, but I see here that back wall recommendation vary between models. So, Can we conclude rigorously it has nothing to do with the port and it's a misconception? Anybody could, for personal education point me to good reading material on the subject so I could understand?
Folks have already covered your question pretty well.
I do want to point out the following
Bass waves are huge and in general a few inches of movement pales in comparison to the wave size. (think wave size to port distance on front vs on rear, bottom, top, side or wherever.)
25hz wave 45ft/13.72m
50hz wave 22.51ft/6.86m
65hz wave 17.31ft/5.27m
80hz wave 14.07ft/4.29m
125hz wave 9ft/2.74m
200hz wave 5.63ft/1.72m
That said by moving the speakers you change the SBIR dynamics quite a bit. You can create cancelations and of course mitigate them by placement. This has less to do with the port than it does frequencies usually produced solely by the woofer. (above 80hz)
By moving a speaker closer to a wall or further from it you change SBIR frequencies. Here a few inches can matter as to which frequencies are reduced due to the cancelations from floor/sidewall/ceiling/front wall bounce as many times these are in the 80-200hrz zone.
A speaker 2ft/50cm from side wall,front wall and having a woofer 2ft/50cm off the floor will have a huge cancelation around 125-200 Move it 1 foot and this will possibly fill in quite a bit for example. Good reason to explore making these three distances different if possible.
The speaker also has some amount of Baffle Step compensation involved in the design. From 0db to 6db is normal. 0, or maybe 1 or 2 db for an on-wall, 3 or so for many speakers and 5-6db for a hifi speaker designed to be showcased out on stands and still have full bass and lower midrange.
Moving a speaker closer to the wall that has 6db of BS built in likely will make it sound thick, likewise moving an on wall away from the wall will thin it out audibly.
This is all mostly happening from about 100hz-500hz give or take depending on design and will change the sound very drastically when critically paying attention and has nothing to do with the port design at all. Yet one may perceive it as a port design issue unknowingly.
Really the measurements one takes in room will tell the story. You will easily see what is what using REW, the moving mic method and a decent mic.
just wild guess, maybe it's coz the rear port are more likely to have a smooth and larger opening, as opposed to front ports which tends to be slotted and cause more turbulance/resonance problem?
There is not really inherent reason a slotted port will have more turbulence.
The velocity of air movement in the port causes this turbulence and once it reaches a threshold that all designs have it will chuff.
Generally a smaller diameter port does chuff sooner and more often.
There is a balancing act with port size in a monitor speaker as the box is small and often the required port for a low tuning is long if the diameter is a larger one. (smaller diameter = shorter length for same tuning vs a larger diameter)
This a problem on another level as that a long port is often more likely to have a resonance that is in a frequency region that internal box absorption material can not affect/reduce. There are other ways to reduce this resonance of course, if relying on wool or similar you want that resonance above 1khz or higher so the material can affect the sound waves, that material can't do anything for a 500hz/750hz ect wave.