Experience with stacked subwoofers?

@bobelton What is your low frequency goal?

1. The addition of the subtle extra low frequency from instruments and effects found on so many modern recordings? 

2. Reinforcement of your current speakers woofers?

Speaker positions are often in an area of the rooms null. A null can deplete the bass output of most any speaker even those equipped with powered subwoofers.

This from a brochure ["REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)’]

Since -6dB Sub-Bass Systems (woofers) roll off so dramatically they simply do not excite a rooms standing wave bass modes which allows them to be positioned most anywhere in a given room. The trade off is the rated extra low frequency rolled off output is barely audible regardless of how many woofers are used. 

Despite this limitation and for what ever there experience, there are many satisfied users.

If your goal is number 1 you’ll need to be shopping for actual -3dB subwoofers.

Here a subwoofer array (distributed bass array) is typically corner placement of four cabinets were their output loads the room equally mostly eliminating the room standing waves. The extra low frequency presentation is quite stunning in my in home experience.

Using one to three -3dB subwoofers requires more precise positioning.  All the best in your search. 

@lonemountain  I spend the afternoon setting up the first four REL S/510 of my six-pack line array....and damn does that sound good...and I still have some fine tweaking to work on.:)

And I am an "old people." 

After the ten minutes spent locating my rooms two strongest standing waves I position the two twelve year old subwoofers within. The twenty minute Auto EQ runs while a Sweep Tone CD is played through the main speakers as each subs fourteen parameters are adjusted to both the speakers and the room simultaneously. Followed by a minor five minute manual tweak to personal taste all from the listening position, close the laptop and done. 

I'm not only old, I'm a lazy idiot. I still have that silly Columbia 'Sneakers' LP - that 'Cosmo' (Kramer) track, jeez. Best of luck with that. 

@hifidream    Do you know of a published manufacturer frequency response ( ex. -xdB@xxHz ) measurement for Kinergetics SW-800's?

I get led to an SW-800 User Manual residing at Hifiengine but I'm unable to log in.

@hifidream I appreciate your ​taking the time to respond.

I think I've seen that 12Hz figure somewhere. I was trying to get an idea of their output level at their claimed low frequency.

By example SVS claims your SB-16's -3dB@16Hz, while Audioholics measured them a very close -3dB@18Hz outdoors. 

Regardless, the description and your satisfaction of your systems low frequency performance must be stunning. Enjoy.