If the OP were to install his KEF Q950 flushed into the wall, what will be the estimated Transition Frequency?
The transition frequency is the point where the speaker changes from monopole to omni pole. So above the transition frequency the speaker radiates forward, below the transition frequency it transitions to radiate in all directions equally and so is omni-directional. So this means the forward radiation has to be halved. So unless this is taken care of in the crossover the speaker will sound thin. The only way to do this with a passive design is to drop the impedance. This is why amps that can not handle 4 ohm loads are not really fit for purpose.
Now speakers are designed to be free standing, unless specifically designed for another application.
Now if you take a free standing speaker with BSC in the crossover, then the speaker will be a forward radiator only, it obviously can not transition to being an omni- directional radiator. The wall prevents that. So if the speaker with BSC is placed flush with a wall, then the BSC compensation will cause a rise in output forward below the transition frequency. That is elementary logic.
Now the KEF Q950 has a front baffle width of 9.6". So the forward radiating F3 point without baffle step compensation calculates out to be 475 Hz. So if the speaker is placed flush with a wall the response will rise by the amount BSC boosted the response below that transition point.
The dips and boosts due to reflection from the wall are a much different issue, and not easy to predict. That is where you need to measure. But there are two separate issues facing the designer of an in wall system.
The issue of BSC in speakers is a difficult problem and the principal reason why speakers can be so difficult to place and sound so different, in different rooms.
The calculation is based without reference to the room. I can confirm that BSC does ideally need to be adjustable. So that is why I made the BSC of my front three speakers continuously variable. It takes an active design to do this. Not only that, but the BSC is handed to the top 10" driver in each of the mains, and also blended with the sub/LFE signal. In the center the BSC is handled by a separate driver from its own amp. This off loads the drivers that would otherwise be burdened with the added power required for BSC.
I can confirm that the BSC setting for the speakers are very different between the old room and this new room.
The surrounds which are my old location monitors dating from 1984, are passive but have two settings for BSC. I did this because of being on location in a lot of different rooms. The rear backs, are able to have the BSC altered by virtue of the placing of the active crossover, and were designed to be in shelving but come out proud of the shelving.
The take home of all this, is that the DIY designer constructor can design for application. This in fact gives the home designer a huge advantage over the commercial designer. I really mean that advantage is a huge chasm, as speakers are designed for the space and adaptable to new ones.