It makes sense that Balance would outperform BL 17. Updated DSPs, drivers and I am assuming using scaled back (or even the same) technology as on the higher end products with room compensation like the BL 50s. Clearly, based on the size of the Balance it is likely limited to lower volumes. But there is no reason why 2x 5.25 inch bass drivers won't outperform a single 6 inch driver.
The BL 17 probably is better suited as a surround speaker. The BL 18, is clearly more iconic.