Again, that’s not what I was suggesting: that forms a straw man argument. I never suggested using DSP in the feed to the main speakers - I tried that and, like you, for music reproduction, I didn’t like it. I was suggesting DSP only for the signal for the sub.
For most rooms less then 7m in the smaller dimension, then when installing a sub, a DSP or other notch filter in the signal to the sub (only) is the only way to avoid excessive excitation of the room mode resonances. If the sub has the same output at these frequencies as it does at other frequencies excess excitation of the room mode resonances will occur, this is simple physics and is unavoidable in any normal domestic room (note that, for instance it doesn’t apply in anechoic chambers above their lower cut off frequency).
(Caveat: If you have a room larger than 7m in the smaller dimension or a sub that has very limited bass extension, then excitation of the room mode resonances may not be a problem for you. If this is not the case, then the DSP or other notch filter will improve the sub/room integration.)