Plenty of reasons why it's superior, you want each of the three channels to be independent, with no EQ effecting the others.
IMHO it is not that clear-cut, and I personally agree with
@harkpabst on this. Let me explain why.
The combined L/R correction is bad as the left & right will have different responses.
You can have either combined ("stereo") or individual channel ("L/R") PEQs. So different correction for left and right channel is possible already.
You don't want to use a PEQ for the left & right for it to effect the subwoofer
You don't want subwoofer PEQ to effect the left & right.
Can you explain exactly why not?
Note that we hear a combined speaker+sub response at the listening position anyway, and actually it is easier to get a smooth total response if you apply room correction to the combined speaker+sub response. On the other hand, if you had the ability to EQ the sub and speaker individually there's really no guarantee that a smooth sub response + a smooth loudspeaker response would always result a smooth summed response at the crossover. From that perspective you could also argue that it is a "superior" approach to EQ the combined speaker+sub response.
Let's think of it another way - integrating a subwoofer is like adding another "way", e.g. adding a sub to a 2-way system makes it effectively a 3-way system. If EQ-ing the sub 'way' separately is always "superior", why is it OK to EQ both 'ways' in a loudspeaker with a single PEQ channel?
You may also note that many professional (studio and live sound) subwoofers have integrated crossovers with LPF applied to the sub and HPF applied to on the line outs. In such system you just connect the source to the line in on the sub, speakers to the sub line out, and as a result you can only EQ the total system response. It is basically equivalent to the approach that the WiiM devices use - again showing that it is a very valid approach.
The speakers may not need any correction at all, so you just want sub EQ only.
Or if you run L/R through a minidsp or similar then you want L/R unaltered but then you only use the wiim to correct subwoofer
In that case you can apply PEQ only under the crossover frequency. You can do this in the current implementation already.
Or you use L/R pre outs or subwoofer pre out through external PEQ so you only want to correct specific channel. Ie if you have a antimode for sub you want the subwoofer channel uncorrected.
You can either just apply PEQ only over the crossover frequency or disable the "Subwoofer Calibration" option in RoomFit. You can do this in the current implementation already.
To clarify my position, I don't think it would necessarily be bad to have separate EQ for the sub, but I see it just as a different approach to the same problem. There are some benefits to either approach, but both can also achieve similar results if applied correctly.
But there is one thing I'd personally like to see - I'd like to be able to measure the Left speaker, Right speaker and Sub responses individually with e.g. REW. This would be very useful to be able to manually determine the delays and phase settings when optimizing sub integration. But I realize this is probably difficult to implement with what is in effect a stereo playback device.
