Placement of Dual Powered Subs with Tower Speakers

Yes, but not the same way.  When you place your main speakers you place them for symmetry above all.  In 2 sub setups symmetry is often a bad idea.

You want to place the first sub as ideal as you can, then place the second so it fills in thee nodes left behind.

Consider the Room EQ Wizard’s room simulator as a useful guide.

the first sub is ideally placed, second sub facing straight ahead, or possibly on the side ?

By placement I mean location in the room. That takes some effort.

Turning it to the side, or away from the listener, etc. usually has very little benefit, so orient them in the most aesthetically pleasing manner.

You want to locate the first sub so as to have the smoothest measured response, with minimal anti-modes (big dips) and small peaks. Then locate the second sub in the room so as to complement and smooth it all out. 

Turning your sub around won't really help this. :)

I've been using a single S/5 Sho for the past year, and it is carefully placed and sounds very good

My suggestion would be deliberately placing both subs with asymmetry in mind. For instance, one might be on the front wall near the left corner, and the other might be on the right wall near back corner, but a different distance from that corner than the first sub is from its corner. (You want to avoid running that second sub - the one furthest from the mains - up very high in frequency, because you don’t want it passing upper bass energy loud enough to betray its location when the music is playing.)

The general idea is that each sub interacts with the room differently and produces a significantly different in-room peak-and-dip pattern, and it is the SUM of the two dissimilar peak-and-dip patterns that matters. Nothing against the strategy Erik recommends, his may work better; either one will result in a worthwhile improvement.

If both subs have phase controls, there may be yet another strategy you might try.

Whatever placement strategy you use, imo the exact position of each sub is less critical than when you only have one sub.

Duke

thanks, I currently have one in between the speakers, though closer to one side, the second hasn't arrived yet.  The one is crossed over at about 40hz...looking forward to this, thanks again

I just went through what you are going through and if you check out my post you will read a lot of different things. As you, I received the first the day before the 2nd and got it integrated pretty easily. Adding the second proved challenging for me. I was able to talk to customer service with REL and he assured me that when it comes to phase once you have one set correctly the other sub will be set the same. Get one tuned in, unhook the Speakon and hook up the other and get it dialed in. Don't try to limit yourself then it comes to setting crossover and gains and they may not necessarily be the same; one subs placement may require a little more gain. I have one side of the front wall that opens into another room so it's not ideal. My subs are placed asymmetrically as one is placed in a corner angled as REL suggest and the other is placed inside my right channel bookshelf over a foot from the wall facing straight out. It's very listenable now and I know with some minor adjustments I'll get there. I'll help any way I can if you want to PM me.

The general idea is that each sub interacts with the room differently and produces a significantly different in-room peak-and-dip pattern, and it is the SUM of the two dissimilar peak-and-dip patterns that matters.

Riiiiight..... so...

Nothing against the strategy Erik recommends, his may work better; either one will result in a worthwhile improvement.

What exactly do you, @audiokinesis  ,think the difference is, in setting up 2 speakers between what we are recommending?? , because I can't see it. :)

@erik_squires asks a really good question:

"What exactly do you think the difference is, in setting up 2 speakers between what we are recommending?? , because I can’t see it. :)"

The difference is admittedly subtle. Just to recap, your suggestion:

"You want to place the first sub as ideal as you can, then place the second so it fills in the nodes left behind."

And mine:

"deliberately placing both subs with asymmetry in mind."

Both suggestions end up with asymmetry; but there is a minor difference in the starting points: Yours starts with optimizing for one sub and then adding the second, and mine starts with planning where they BOTH will start (which is far apart, and asymmetrical).

My partner Jim Romeyn (who manufactures the DEBRA system and is probably better at set-up than I am) recommends your approach, complete with ye olde "subwoofer crawl".

Duke

planning where they BOTH will start (which is far apart, and asymmetrical).

OK, so how do you get to this asymetrical plan? Do you use a room simulation similar to REW's ?

Erik asked: "OK, so how do you get to this asymetrical plan? Do you use a room simulation similar to REW’s?"

You could.

In practice the room’s layout usually imposes constraints, and from there one just applies the basic principle of maximizing spacing and asymmetry. Like I recently did a layout for someone who wanted to avoid wires crossing walkways, and one quadrant of the room was off limits. Those were the constraints, and from there it was pretty straightforward.

Duke

Erik wrote:  " Well, color me surprised this works as well as you have experienced!! It feels a little too much like relying on serendipity for me to have every thoguht to try it this way. :-) "  

I have nothing against measurements and modelling and use both extensively myself. But not everyone has access to such, so I have suggestions which do not require them.

Duke

If it works, it works!

@audiokinesis 

I appreciate that Duke

Thanks to both...the only placement obstacle that comes to mind is there is a single available electric outlet though I think it’s in a good place. Thoughts on long power cords? Thanks again 

The comments thus far seem to presume forward facing woofers. Currently, I have SVS subs that fire down

See Acoustic Fields website for lots of tips on subwoofer placement

My REL has a forward firing active driver and down firing passive 

jl35, I think both audiokinesis and erik are right from the perspective of diminishing nodes throughout the room. But, there are other considerations particularly if what you care about most is the listening position. 
I would like to add several other points. The harder you push a subwoofer the more distortion you get. The placement that is most efficient is in corners, next is up against a wall. Timing is very important with subwoofers. It is important that the wave front from the subwoofers gets to you at the same time as the wave front from the main speakers. This is easily demonstrated with kettle and bass drums. You feel this more than hear it. In a system that dose not have digital bass management the most likely way for you to achieve this (but no guarantee) is to place the subwoofer equidistant with the main speakers from the listening position. The only practical way to do this with two subwoofers is to put them between the main speakers with all speakers on the same radius from the listening position with the subs right up against the wall for efficiency.
This will obviously not produce the best nodal behavior throughout the room but it can provide the best performance at the listening position. Instead of "crawling" to position the subwoofer you "crawl" to position the listening position! Walk forwards and backwards from your listening position and find the spot with the best bass balance to your ear. You will be surprised how much the frequency response will change with just a few feet of movement. Use a song with a lot of synthesized low bass or undamped bass drum. The first cut on Brittany Howard's new record "Jaime" has a great undamped bass drum, perfect for this use. You can feel the air move in the room. 
It is also a good idea once you have everything set up to reverse polarity on both subwoofers to make sure nothing is inverted. The polarity with the most bass is the right one. 
It is always a good idea to use a high pass filter on the main speakers. If you do not already have a high pass filter you can make a 1st order one just by placing a capacitor in series with the positive leg of the amplifier's input. The value is chosen based on the crossover frequency and the input impedance of the amplifier. You can easily find the math online. 

In practical ways, Duke's own method is not very different than my own.

I think duke is more conscious of the fact that most of us can't really place subs wherever we want to, which is true.

So, the reality is you place subs were best you can, but what happens next?  You have to set up the crossover frequency and phase, and there's just no way to do both at once, and as Duke points out, having different phase or, by extrapolation, different low pass points and levels may be needed.

So, I think we are all circling the same tree trying to catch the same solution.

Thanks to all...second sub arrived today. After initial set up with one in between mains but closer to left, and the other outside of right speaker, and closer to side wall...after fiddling with settings it is sounding quite good..know it will take more time to get it just right, but it is already quite good...hopefully in a few days a friend will come and we will move the subs around some and work more on the settings...