Thorsten,

The problem with your idea seems to be crossover frequency. In order for the subwoofers to be sonically invisible and to not damage the imaging, assuming the use of digital media, you are goingto have to cross over to the main speakers below 80hz and eventhen you have made no attempt at time alignment (essential for transient response IME). If crossing over this low down, your satellites will still be exciting the 2nd harmonic of the main room node along with the vertical axial node.

Without care the results will be disappointing in a number of ways. The implementation of digital delays is the only way currently to get around the issues highlighted above.