You are definitely on the rite track... It is pretty much identical to my desk setup EXCEPT, I swapped out the cbm170s for a set of 200s, then swapped the 200s for a set of Wharfedale EVO2 08's, and while I love HSU subs, I bought a matching wharfedale 12" sub {all in matching light maple}. The Evo8s just sounded a little better while rite in front of my face, but the biggest reason for me changing was the aesthetics, the ascends sound awesome but they look so cheap with the vinyl finish.. In a theater they are awesome, but rite in front of me at my desk, I wanted something wood with a nice fit and finish...
If you are interested there is a set of evo 08s in rosewood on ebay for $295 buy it now, they wont last so if you are interested I would jump on them.... Also a set of evo 2 10's {a little larger but also sound great} in light maple for the same price from another seller.. Just search wharfedale Evo on ebay...
I also use the same emo mini amp for my desktop, I used to use a dayton apa150 which also sounded good {I would put them in the same league when it comes to sound, the apa is a little cheaper and has a little more output, but is harder to fit on your desk, which was my reason for going to the minix....
Next is the dac, you chose the rite one, I own a couple, and love them...
Next for the sub, HSU would be what I pick, the stf2 is a good sub for the money, to get better you would have to spend more or build it yourself....
Hope this helps, if you care about the looks {which you may because you have a really nice desk} look into them wharfedales on ebay, they look new... If you don't want used the wharfedale diamond 10.1 and 10.2 s are really nice bookshelfs too....
Just trying to give you some options, but as is you have a great system laid out there. You may want to do some measuring the cbm170s are a biggish bookshelf, the 200s are much more at home on a desk, and sound just as good, and better with a sub...
As for crossing its simple I use FMods
Amazon.com: FMOD Crossover Pair 100 Hz High Pass: Car Electronics between my dac and amplifier... then play with the subs crossover until they melt together with no seem... I tried 70hz, 80hz, 100hz,, and 120hz high pass fmods, the 100s sounded best... With my towers the 80s sounded best, so each system and room may be different, 100 is a good place to start... Plus doing this will allow the amplifier to focus its energy creating the mids and highs leaving the lows to the subwoofer...
I can load up some pics of my desk top if you like..
PS- 1 more thing to consider, I think this would make an awesome 2.1 system...
http://www.musicdirect.com/p-42451-focal-chorus-sw700v-subwoofer.aspx
http://www.musicdirect.com/p-41245-focal-chorus-705v-bookshelf-speakers-pr.aspx