I think you are missing one important point - the real benefit of corner placement is that you get direct and first order reflections pretty much aligned - if the SUB is out from the wall the reflections will be delayed in time, these reflections will be difficult to control using DSP.
You are right about dips, a good room correction systems would preferably only lower the peaks and not try to boost the dips, however with the added gain from the corner you have a lot of headroom to shape the response by "cutting away".
Having studied this a bit, I'm now leaning towards a Sub (actually Woofer) system in a closed box with low box Q (as low as 0.5) using a driver with low MMS, Rms, Le and high BxL. So not a sub unit in classic sense
As mentioned I have heard this work with the Steinway Lyngdorf system, so I'm very eager to challenge the conventional wisdom you are referring to
