Tom,
What I used for specs was simply an example. Only truly excellent systems will have response figures in that neigborhood. Usually the ones over $1000.
One must remember that most subwoofer systems are ported. Ported systems roll off response below their tuning frequency at 24db/oct. A ported system, unless tuned extremely low, will not play linear much below 30 hz (as most are tuned between 28hz-35hz) in most cases. The PSB appears to have excellent characteristics. Also remember that room interaction/placement will play a big role in real, deep, low end response, below 30 hz. Those tests are done in a very large room. Actual in room response (2000 cu.ft. or less) would deeper I am sure (depending upon placement).Try to avoid systems with spikes in response, unless you will be crossing the sub over below that frequency. Even so, a sharp slope of at least 18db/oct. should be used.
If a sub is at +-3.5db or +-3.2db do not be concerned. That is still very good.
The B&W ASW 675, Velodyne CHT-15, M&K V76, Energy S12.3, or Velodyne DPS-12 all have good response #'s as well. The PSB, however, still has an edge in linearity.