What would you upgrade next?

What are your goals on the upgrade?

Wow! tough call:
I would look at one of these options.
Upgrade the monster for a ps audio 300.
Upgrade your speaker cables to MIT 750 magnums.(or better)
Upgrade you amp to a sunfire signature.
Just a thought......Good Luck........Vader!

Agree the Monster conditioner needs upgrading...
But seriously, look at that B&K pre driving a DNA-2 Deluxe - that just does not make sense to me. The pre should be the best component in any system. You're compromising the whole rig by driving it with a marginal front end. That SACD is choking. ATI for surrounds is OK I guess, but only OK within the function it's performing.

biggest improvement for you you right off the bat, given my experiences with hearing the vast majority of audio enthusiest's and even audiophile's systems I've come across out there, IS DEFINITELY THE SET UP/ACOUSTICS!!! 50-60%+ of the overall sound you are getting(or not getting) is from proper speaker/seating set up and acoustic considerations in the room! Most fail misserably here, and get poor overall sound with even the best gear. If you have this down, where the speakers are ALL giving you even/flat frequency response(rare...trust me) from the seating possitions, all the speakers are in phase with each other, especially the subwoofer(most have way off), and you are properly dealing with the other acoustic issues in your room, you will improve results greater than any equipment changes you could possibly make!...no understatement.
After that, you should strongly consider upgrading that rather beamy, low dynamics center channel, for the better MC1 that Thiel makes. The SCS3 is the weak link with those speakers. Also, make sure you're crossing over your mains(that's right, the CS6's) at 80hz for movies, and letting the sub handle bellow that! Really, full range music speakers can't handle DD/DTS bass on their own, and NEED to be crossed over higher, even though they play full range.
If, like most, you can't get proper set up results on your own(simply measure with a sound level meter, or listen to ONE SPEAKER in mono/summed stereo to see how your speakers really sound), then have someone help you who's skilled enough(like me) to help you do it!
Otherwise, any upgrades, like better powerconditionsers(PSaudio and better), or better cables will only go so far.(but will help, yes).
good luck...if you get the set up and acoustics right, you're most of the way to world class sound...

Lthkeepr makes a very strong and valid point. Treatments are as good as, or better, than many upgrades. Also, agree with upgrading the Monster. Might I suggest an Audio Magic Stealth - money well spent, IMHO.