A lot of the above can be scaled up for studios and mixing rooms, where room sizes mean that bass-end standing waves predominate. 'bass trapping' is very much required to get anything near an even response. Also within a room, as well as getting louder notes, standing wave null points become an issue as well, as if you are in the wrong position, notes can simply disappear! So thick rockwool/mineral wool or glass fibre panels are used to help absorb the bass end - these are especially effective in corners.
The open-cell foam panels you often see in studio pictures are effective at taming higher frequency sounds - those with regular pyramid patterns are also part-diffusers (for reducing direct reflections form walls). But they need to be at least 7.5cm thick before they start to have any effect on the bass end. Again, setting them off the wall helps them be more effective.
Yes, open backed cabs disperse the sound over a wider area, whilst closed back ones, especially 4x12"s, can be rather 'beamy'. On any multiple speaker cab with speakers close together, you start to get into speaker coupling - that makes them more efficient - which is why a 4x12" is louder than a 1x12" using the same speakers and with the same signal going through it. You are also starting to get into the sound wave interference effects that make line array PAs so effective at controlling the spread of sound - wide horizontal angle to cover the audience but narrow vertical spread so you don't get any primary reflections from the roof or floor to muddy the sound. So the sound output from a 4x12" will always be a lot more directional than a closed-back 1x12", which in turn is more directional than an open-backed 1x12".
