Small footprint subwoofer

@ soix  I am not saying there is no instruments lower then 30z. There is but very limited and the experience is more of a feel then hearing it. 88 keys piano range can go 27.5hz and pipe organ can go as low as 16hz. A bass guitar has or a double bass has the lowest range at 35hz. For the kind of music I listen to my not required to go that low.

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. 

Love the REL T5x, perfect invisible extension for Wharfedale 85th Anniversary Lintons.

I like REL also.   T series are really good subs for the money.  I have a 15 plus year old r305 that has been thumping along every day....   

I have a REL S/510 which I like a lot. I don’t like looking at subs so I wanted the small form factor