Remember that +/- 3 dB gives the sub a 6 dB window of difference. The low end roll off looks about right, although their high end roll off doesn't quite match SVS's specs, but this could be due to the way the crossover was setup in the review. It's a sealed unit, and I haven't seen measurements for any sealed unit which were flat to 20 hz, unless it was EQ'd. There is nothing surprising here.