Experience with stacked subwoofers?

Go and develop a relationship with an audio component dealer. I doubt mine would ever sell me something that wouldn’t work for my room and system. 

Whenever you “stack” speakers of any sort you alter dispersion in the direction they are stacked.  Ever seen/heard of a sound “column”?  This is exactly the principle, stacking shortens vertical dispersion.  This is used effectively on tour sound system to shorten vertical output ( determining how far the sound is “thrown”, but maintaining a wide horizontal.  Stack the speakers side by side, same thing, shortens width and maintains vertical.  So in controlling bass in a space, stacking is a very common method that has been used by experts for many years. helpful in a very reverberant/reflective space.  It is not something REL developed.  Any competent speaker engineer would be aware of this principle.  This is the argument for and against these long tall or wide arrays of similar drivers, such as using multiple tweeters next to each other in one cabinet- it creates dispersion lobes that have to be managed;  its not hard to hear those lobes moving back and forth across the array.   Advanced DSP can alter this inherent principle but its not easy or simple or something you can do on your own at home.  

I've been part of designs in studios that use linear sub arrays to narrow the width of dispersion from the subs. 

Brad

I have a pair of REL S/510, and just bought two more pair on dealer demo closeout (the S/510 were just discontinued). I will be dialing in a six-pack soon. I have a four-pack going now (which require some more tuning) and am waiting for the dealer to deliver the last two and help with setup.

I would love to hear how that goes when you get it all dialed in. 

Yeah, me too. LOL I';ve been using a stereo pair for years and they sound fantastic.

I'm about to do a massive upgrade on turntable, rack and TT isolation also (I've had a windfall and plan to spend some of it on fun stuff). I'll get the six-pack dialed in first though.

I have 2 Rel 510s in my setup and love it. For me, no need to stack up I am totally satisfied with my sound quality.

At AXPONA back in April, Rel demonstrated a line array of 510s with a pair of Wilson Sabrinas (I think... I was paying attention to the Rels).  Rel not only demo'd the effect of the full line array on/off, but also demonstrated the effect of running just one 510 per side, two 510s per side, and the full stack of three per side, sequentially.

I was very impressed - the full line array added significantly to the size of the soundstage and improved the imaging in ways that I would never have anticipated.  The effect was, of course, most significant with the full array but was present with just two of the three Rels engaged on each side.  Of all the experiences I had at AXPONA this year, the Rel demo was the one that stayed with me... I've been thinking about it ever since the show.

Consequently, I've begun to accumulate Rel Carbon Specials to add into my system. I have 4 of the 6 on-hand (in boxes, not yet set up), so can't provide impressions yet but am certainly looking forward to integrating the six-pack.

Have been using stacked subs in my HT room for almost a year now. Still sorting the room out, and don't have long enough cables to put them in the proper place. 

To sort out L/C/R speakers, the subs had to move, so they go stacked in the corner of the room. Yes, ran DSP after. Both subs have footers under them, to isolate from each other. 

They sound just fine, get all the bass I could ever want and then some. Once stacked, both needed the gain to be turned down. Think it's a little cleaner, but these are movie subs, not musical. 

@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. 

I've seen a Youtube video on this very subject. I found it when I was researching Rythmik Audio subwoofers.

If you have decent speakers you only need one subwoofer maybe two.

It’s a gimmick from a marketing standpoint. Not sure how meaningful the changes and I’m not hearing much on what it does up above and there seems to be limited enthusiasm.

Why not Stack tweeters and mid range drivers.

I stack my dishes and silverware and that really works good.

A Ripole Subwoofer might be the one to investigate. 

Everyone hears differently and has different values regarding audio purchases. I've heard several systems with REL 6-packs. I didn't like the sound of any of the systems I've listened to with 6-packs. I actually own dual REL subs, so I'm not being a REL hater.

Thanks for all these responses. I have two Rel t7s in stereo in my current set up, which were one of the greatest improvements made. In my own personal experience, I have found subwoofer replacement and even toe-in to make a significant impact on the quality of the base that I hear from the listening position. Not just the amount of bass, but the ability to hear distinct differences in the notes from one to the next. Having experienced a dramatic difference with that, I’m imagining that adding a height dimension would also be significant. So I’m intrigued with the idea of stacking three subwoofers, each tuned to accomplish something slightly different. 

A good friend of mine has a pair of GR Research, Double Trouble open baffle subs. 

One stack per channel.

Probably the best sounding subs I’ve heard.

Well, tomorrow is my day to try dialing in my REL S/510 four-pack. I’ve watched John Hunter’s Youtube about doing that setup for a six-pack. When my dealer delivers my last pair they will send their expert on setup to help get me sorted, so I’ll see how close I get to what they think is good, at least up to the point of working on fine tuning. I even have the Sneakers Track 4 audio they recommend , ready to go on Qobuz.

@simonmoon: The GR Research/Rythmik Audio Open Baffle/Dipole Servo-Feedback Woofer is definitely the best sub I’ve ever heard. The one in your picture is not actually a stack, it is the 2-woofer version of the sub, also available in a 3-woofer version (the dipole cancellation inherent in open baffle subs makes a single 12" woofer per side insufficient in maximum SPL potential). Most audiophiles will never hear it, as it is sold as a DIY kit. However, GR Research has a couple of cabinet makers who will build a pair for you (for a price, of course).

Advice for potential buyers of the sub: The Rythmik Audio A370 plate amp that comes with the kit is solid state, so is of course optimized for a 4 ohm load. The woofer used in the sub was optimized for open baffle use by Danny Richie, and is available in both 8 ohm and 16 ohm versions. Danny routinely sends the 16 ohm version with the kit, so that 3 woofers may be used with the A370 plate amp (the combined impedance of 3-16 ohm woofers is around 4 ohms). However, if you intend to go with the 2 woofer version, get the 8 ohm woofers rather than the 16 ohm. The 4 ohm combined impedance of 2-8 ohm woofers maximizes the power output of the A370 plate amp.

Having experimented with different subs and arrangements in-room  I can say with some authority that the results you get may surprise you. And it can go either way. My experience supports the fact that if a single sub is good, then two is way better, and not stacked on top of each other in one corner. While that gives you more output, it won't help control room modes and give you even bass response thru the room. Separate the two subs and treat them like they are stereo because bass is not monophonic. It does not mean they have to mirror image in terms of positioning but try running them on either side of the screen or tv out from corner along the front wall assymetrically and run some measurements. Keep playing with positioning until you are happy with the results and then do some DSP on the remaining peaks and you should be in pretty good shape. That second sub will even out the response and give you 3 db more sub output, helping perhaps cut down on low freq distortion a bit. Well worthwhile expense to add the second sub.