Good for you, I'm sure you will be very happy with the LS27.
As for the Depth subs. I have run both stereo and mono set configuration and both coaxial from the pre and speaker cable from the main speakers.
My best results with 2 Depth subs was running both mono at the side walls of the room about 14" away from the wall. I realize this is not possible in most living situations though.
Virtually all rooms have bass issues. With them in stereo you are not doing as much to eliminate nulls and modes that exist since each sub is sending a different signal. With them in mono you are increasing the signal from different directions so as the sound waves move around the room independently you will have a better chance to correct the problems that exist.
I don't know how much flexibility you have in your set up but try not to put the subs behind the front plane of the main speakers. For me that proved to be the worst position for them.
If you haven't already try the tried and true sub crawl method. Place them at your listening seat position and crawl around the room with music playing. You will quickly find where the best low end is located. Take the best 2 if possible and place the subs there. If one is not possible take the next best.
When I was fortunate enough to upgrade to 4 Depth subs the front and back locations made a huge difference. They are all run as mono. I ran left and right coaxial cable to 2 of the subs then, i continued to the other 2 subs with a mono signal via coaxial again. I did find out one thing I had not expected with the 4 subs.
We have all experienced that different pieces of music have different levels of low end. With the 4 subs if I play a piece of music that is mixed with more or less bass than I care for, I can make a tiny adjustment in the sub volume level that is behind my listening position and the only place the change appears is at the front of the room in the sound stage. I'm not sure why technically that works but it does.
Some feel the signal coming from the power amps to mains then subs is better since you are sending the exact same signal to all speakers. My experience has been it didn't matter. I would suggest try all the options of cabling and location. When I was experimenting I would often use less than ideal cabling run the most direct route for testing. It makes for an ugly criss cross of wires but easier. Especially if you go the coaxial direction.
Let us know how it works out.
