Need help placing a second subwoofer.

"...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. 

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?

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

+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.