Need help placing a second subwoofer.

+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...  :-)

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.

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.

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.

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.