The Plus/2 should be capable of this (with the 12.3 drivers, better) performance. It's a ground plain measurement taken at 2m from the the subwoofer.
This is the Servo-15 measured under the same conditions.
As you can see, they're both pretty flat, but the plus/2 has roughly 10dB more output across it's frequency response.
As I said earlier, those PB-12Plus/2 measurements are from an older model, using different drivers. The new drivers are suposedly better, so you can expect better numbers then those.
SheepStar