Need help placing a second subwoofer.

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. 

"...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...  :-)