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. 

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 

Thanks for the response. The room null I am have right now is above the xo point anywhere around 90 to 120hz. Most likely is due the position of the speakers and the in room response. The node is at around 60hz so I have corrected most of the issue with a inline 12db/slope 70hz crossover for the speakers and the sub setting at around 50hz. Theoretically this would be a 6 db less at the xo point. On the contrary, the null that I have is probably more complex to fix. Honestly, this is a bad room dimension but it is the only room in the house I can relax.

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.