Please use an online translation program, it is hardly possible for me to understand the meaning of your sentences.
A flat frequency response roll-off in the low frequency range is better than a steep drop-off, that is true. But even better is a linear frequency response (constant phase). Best without additional excess phase by filters like for example around a crossover frequency.
The CubeMix speaker shows a high pass behavior below 1kHz and a low pass behavior above 2kHz, so it works like a bandpass around the 1-2kHz frequency range.
This leads to a relatively high group delay in the range 100-500Hz.
A good 2-way loudspeaker has a linear FR in this frequency range and shows only a small off-set in the GD depending on the crossover frequency and filter order. Therefore the GD of a 2-way speaker in the important frequency range 100-500Hz can be better than that of the MixCube - of course the group delay of the 2-way LS is higher below 100Hz, but the MixCube does not reproduce this range at all and in combination with a subwoofer the GD would be similar or higher than with the 2-way LS.
If one would (and could) adjust the FR of the MixCube via EQ, the GD would of course also massively improve.
View attachment 271693
The only price to pay for this is a higher GD in the lowest low frequency range.
You do realize that you can't just add a delay to a signal and then play or mix that with the original signal.