Need help placing a second subwoofer.

Well I ended up with 4 subs, and DAMN, you guys were right. So smooth now, it's like the room and my head are one.

And yea, Supertramp, Beatles, Van Morrison, my favs back in the day.

1111art, it is fun to play around and experiment but it will never be an optimal situation. The only thing you can do here that makes sense is putting the little sub on top of the big one between the main speakers up against a wall. I am thinking like one who runs his subs up to 120 Hz.
If you are using them below 60 Hz with just a low pass filter you can put them anywhere. In corners is always best as you get an easy 6 dB increase in efficiency. Next is up against a wall for an additional 3 dB.

I personally think all subs in a system should be the same. Next would be dividing it up in pairs say two of one kind in front and two of another in back. 

Crime of the Century, Crisis What Crisis and Breakfast in America are three amazing albums by a group of amazing a musicians and all three albums are beautifully recorded. Should be in every music lovers collection. If you do not have them, get them, you won't be sorry!

M-db
I can’t believe it, but that’s where I purchased my first ’high end’ M&K system back in the 80’s. Jonas Miller Sound! They put on ’School’ by Supertramp, and when that kid screams, it scared the crap  out of me, so I bought their best system then and there! Had it till last year when I gave it to a friend.

Since every system and room is different and your using less than four subs this method works well.
https://www.youtube.com/watch?v=AV3oLLMgS-M

If you decide to use this method and while your M&K is still at the listening position draw a map of your room walk around and note all the areas where the bass frequencies are the loudest. These are YOUR rooms modes.

Place the M&K in or near the loudest mode and the Velodyne in or near the next loudest mode. IME running L/R to each sub (ie mono) will increase the room loading.

For economical custom length long runs of cable try Blue Jeans or Monoprice Cable. 

I purchased a new (M&K) Volkswoofer from Jonas Miller Sound on Wilshire Blvd. in 1979. A friend has been using it for the past 38 years without a problem. 

Have fun with it.

I use mine at the inside of my speakers very close to the same plane as the front baffle of the main speakers. You having two different brands may create a level matching issue. Also, on this arrangement, try front firing and side firing.

Not so fast again QuickDraw:

https://ohmspeaker.com/news/subwoofer-placement-no-you-can-t-put-it-anywhere/

https://ohmspeaker.com/news/for-optimum-sound-how-many-subs-and-where-do-you-put-em/

According to this from a very reputable source with years of industry experience and thousands of customers there is a best setup for 1, 2, 3 or 4 subs that includes front and center if just one.

1111art-

But by what I'm hearing, the main sub shouldn't be front and center like mine is, but closer to the main speakers and probably lightly forward because of time delays of the lowest frequencies, is this correct? 

Correct in the sense that is what you are hearing from people, but incorrect in the sense it is wrong.  

Subs put out low bass. Low bass is very long wavelength. So low we don't even hear it until one cycle. So long by the time we hear it the wave has traveled across the room. At low enough frequency it has time to travel back and forth being reflected and canceling itself before we even hear it. 

Time delays in other words are irrelevant.

+1   millercarbon's statements.

And I'd run those Dali's full range unless you feel the need to high-pass for whatever. Unless your amp is small I think the midrange will do fine with the woofers tagging along and not high-passed. Plus they go low, and I'll bet very well on their own.

Set your two subs in good locations to maximize the number of room nodes, not the volume of only one or two, if you want the smoothest bass. Two corners are a bit suspect, especially if you cannot equalize them.

Your bass should not get louder. You should just hear more of the lower bass and it will sound louder because of that. Plus most well-positioned subs add a sense of air and recording venue.

Have fun and good luck. Add two more subs when you can...  :-)

just put them where they look best.  you wont notice a difference anyway.  

Excellent, thanks! I think I'll start experimenting...
But by what I'm hearing, the main sub shouldn't be front and center like mine is, but closer to the main speakers and probably lightly forward because of time delays of the lowest frequencies, is this correct?

"...making a nice "stereo pair" looks nice and tidy but you may be wasting their potential..."

OK, here's the deal. In most cases you want the subs crossed over at higher frequencies, say 80 HZ. As you tune higher for better response, the sub can be located. So if you put them inside it becomes part of your stereo and helps with the imaging. If your subs are for HT explosions, that is a different deal. 

10" is not lame, you just need more of them. https://systems.audiogon.com/systems/8367 

The thing with subs, placement with the first one is time consuming and ultimately impossible. Because being only one, then no matter where it goes you get great big peaks and dips. The time consuming part is moving it all over the place before finally giving up and living with huge compromise, telling yourself it is really good anyway. 

Second sub placement is a bit easier. Now with 2 subs each one puts out less volume so the peaks are smaller and so are the dips. All you really have to do is put the second one somewhere a different distance from a corner than the first one. This will make the peaks and dips as different as possible from the first and give a more smooth bass response.  

This is where most of the improvement comes from, the smoothness. That is why it matters much less how good a sub you use. It is much more important to use a lot of them. With each additional sub the bass becomes even smoother, faster, cleaner, deeper and more powerful. Each additional sub lets you turn down the volume on the others. So you also wind up with a lot more dynamic headroom. Plus placement gets easier and easier. With four you can pretty much put them one to a corner different distances and be done. That easy. 

place any subwoofer or subwoofers where they sound best as a first effort.  consult the owners manual.   making a nice "stereo pair"  looks nice and tidy but you may be wasting their potential.  usually subwoofers are recommended to be placed near corners for best response without getting too close to the walls to cause peaks in the response.  
behind the listening area is certainly acceptable as long as you refine the position by testing.  too close to walls you will hear an exaggerated peak (one note louder than the others, not good)  
pick your spot and then listen to see how it sounds.  too weak = too far from wall boundaries, too over powering, too close.  usually 12 inch from walls minimum. 

"...I always place them in stereo pairs behind and outside (if possible) the main speakers..."


Same except I put them on the inside.

I always place them in stereo pairs behind and outside (if possible) the main speakers. This significantly extends the soundstage. You want the arrival times to reinforce the main speakers.