I suggest you first consider a single SF Gravis V not because its an SF product rather for the flexibility its accompanying application can provide regardless of speakers its paired with.
Without any personal experience with the Gravis I question it's -6dB frequency response rating. Without having read its application instructions if it has auto calibration it makes for far less manual adjustments. It appears to include four customizable presets and assuming the application offers remote control of adjustments and volume. I've found this combination allows a subwoofer interactive flexibility that I can't live without and worth most any increase in expense.
With the subwoofer placed at the listening position playing a low frequency cadence you should be able to identify your rooms bass modes by simply strolling around. The loudest mode becoming the subs location should provide a single sub adequate performance at the listening position. Later the Gravis V can serve as the master for most any more economical non processing sub. They need not match.
Have fun with it.