Small footprint subwoofer

highend64  The two Double Basses I own the open E string tunes to just over 41Hz. Regardless, its how the environment they're played in or how the producer wants it to sound that a -3dB subwoofer is able to convey the delicate sub sonics. Unlike the empty 31Hz buzz of an unamplified open B string from a solid body electric Bass.

Looking at your setup photo it's quite possible your HSU is located in a middle of the room null which requires a subwoofer to be annoyingly overdriven. It can also greatly restrict its potential for exactly the reasons your seeking, while exacerbating the room mode that may possibly diminish by the HSU's being positioned in.

Its true, replacing your -2dB@ 25Hz subwoofer with the dramatic roll off of a -6dB@ 25Hz sub-bass speaker will not excite your rooms mode and allow it to be placed in the HSU's current null location. All at the expense of the HSU's currently maligned potential sub sonic presentation. 

Find your rooms mode first before you spend a dime. 

highend64  I apologize for not being more helpful. While you moved the HSU slightly and it measured differently is not how to position a subwoofer.

The most effortless, quickest and practical way to find your rooms standing wave bass modes must be done with the -2dB HSU placed at your listening position because its the location you're actually trying to improve the subwoofers performance within the room. Think of it as a sort of reverse engineering. 

Connect one or two long inexpensive RCA interconnect cables (from Blue Jeans  or Monoprice cables) to the subwoofer.

Purchase or download a low frequency cadence tones that can be constantly repeated.

As the tones are being played slowly walk or crawl around your rooms boundaries listening for the strongest best sounding bass. Make a map of these locations within the room as you'll only have to do this once. 

With the HSU positioned now in the most convenient mode location readjust the subs parameters while listening from the listening position first with the bass tones then with music, by ear. There is no right or wrong as to what you feel sounds best. 

This method does not work with -6dB rated speakers since they simply do not play play low or loud enough to excite the rooms modes so there's nothing to integrate.

Room size has nothing to do with the delicate extra low bass. You can hear it in many headphones. So why give up on it with a -6dB sub bass speaker?

highend64    'As far as the position of the Hsu sub, yes it is in a near null position by not quite. That is why it is off centered to compensate the difference. I have the tools to do measurements so finding the best place for the sub is not a problem.'

Just curious. Is this the room location the Rational software calculated or just an interim compensation?

All the best with your situation. Take care.