Interesting. I've always run my subs up higher than that, so I am speaking from that experience. When you run your subs only to 50hz, what are your best practices for OB sub placement?
I've always achieved best results with OB subs a foot or more closer to the front wall than the OB and/or Magnepan mains. Also, using multiple 15" bass drivers with a little larger (wider, deeper) baffles, the quickness of the bass is probably just as good as servo subs as those multiple 15" drivers are barely moving, even at high volumes.
The first OB subs I built over 20 years ago used two cheap Pyle Pro PPA15 drivers per side in H-frames measuring 24"W x 24"D x 36"H. In-room, they were flat to 16Hz, used no EQ to get there, and at peak volume only used roughly 50 watts if that. To this day, those are STILL the best sounding subs I've ever heard at any price, and I've heard a bunch. The only thing that I heard that matched or surpassed them are the MC Audiotech Forty-10 loudspeakers that use two 18" pro drivers per side in W-baffles. And IIRC, they were using a single 40 watt per channel amp to power them.
Cool thing... Well, a few things actually. One, I still have those drivers; Two, Pyle still makes those drivers; and Three, I spoke to both Siegfried Linkwitz (RIP) and Nelson Pass about the use of those drivers in OBs before I ever purchased them, and they both agreed they were a good choice and gave me their blessings on using them in the size baffles I planned on building for them.
For crossover duties, I used an Audio Control Richter Scale Series III with a 50Hz/12dB per octave crossover chip installed (came stock with a 90Hz/12dB chip) and only used the EQ on it to roll off that 50Hz a bit more. For amplification, I used a Carver M-400t amp, and the power LED meter on it hardly ever moved. It was rated for 300 watts at 4 ohms and I had the drivers wired in parallel for a 4 ohm load.