Lowoverdrive, your doing your homework.
Because of all the unique variables you'll encounter with your room, your system, and your personal taste, suggestions can be both helpful and a crapshoot. It seems a few here have peaked your interest in digital signal processing (DSP). In my experience as an audio hack DSP is the most important and helpful aspect of dealing with extra low frequency within a listening environment. Despite the convenience of DSP learning the best placement locations within your room will dramatically reduce room nodes and provide that sense of room lock that, I think, many refer to as speed.
There are two standout manufactures both using their second generations of onboard DSP. The JL Audio Fathom series with its Automatic Room Correction (ARO) uses five parameters of adjustment and the remote controlled Velodyne Digital Drive Plus series with its Auto and Manual EQ Optimization uses eight parameters of adjustment and six programable preset programs.
As wellemj pointed out they both use a listener positioned calibrated microphone, a method of providing low frequency sweep tones, and a very user friendly program that automatically adjusts their onboard parameters to flatten their output at a desired location/s. Unlike the the very capable Antimode these offer the ability to override and manually tweak the auto adjustment. The adjustability of either offer a very high level of mains integration and the ability to satisfy your personal sonic taste in extra low frequency.
Your homework assignment: go to both manufactures sites and study JL Audio ARO and Velodyne's EQ Optimization differences and as a means to compare the growing number of new DSP offerings setup procedures. Then consider purchasing one 10" version of either brand and slave the DSP'd signal to a used inexpensive second subwoofer. The investment in either of the latest JL Audio or Velodyne versions should bridge you through any need to upgrade the DSP sub for some time.
https://www.audiogon.com/listings/subwoofers-jl-audio-fathom-110-pair-available-2018-01-17-home-thea...https://www.audiogon.com/listings/subwoofers-amazing-compact-subs-2018-01-24-home-theater