Ok, on BL symetry it does not score as well as the Seas, but on distortion it's a lot better in the mesurments, and that is way more important than a perfect BL curve. That balanced BL curve is nice to have, has only indirect an influence on the sound, and i don't see it in the response or distortion graphs as bad with the SB42...
And that SW26 is a speciality woofer, with other priorities than the best sound, it should be shallow enough to fit under a fauteuil or so and still sound good. It's a very different woofer than both the SB42 or the L26 and you can't compare it for the same kind of use (as it's not ment for it). If you compare the SB42 and the L26, the SB42 wins with ease for me for a subwoofer if i compare all measurements, not only BL symetry.
But at the end, if you want the L26, use it. It's not bad at all (we are nitpicking here), but i keep my remarks from above. I would not use it for this.