in the link for the blue sky exo2 review, the reviewer liked it more than the HS50 + sub. A lot of people seem to think the HS50 + sub > HS80, since there's a dedicated "midrange" woofer (though there are many that prefer the hs80). For your uses, the HS50 might be a better choice since:
The HS80s are huge, LxWxH: 13"x10"x15" (I don't think those will fit easily on a normal desk)
http://usa.yamaha.com/products/music-production/speakers/hs_series/hs80m/
As for connecting an HS50/80 to your PC, just use an rca -> 3.5mm cable.
otherwise, you'd probably need to buy a DAC that has an XLR out or buy the HS10W (paired subwoofer for the HS50/80; or any other sub that happens to have XLR, fairly uncommon since it's not found on typical home theater equipment) if you wanted to use the XLR connection.
http://download.yamaha.com/api/asset/file/?language=en&site=usa.yamaha.com&asset_id=7262
see page 6 for available connections and page 13 for hs10w subwoofer (typically runs ~$300 from quick google)
http://www.monoprice.com/products/search.asp?keyword=rca+to+3.5mm
rca to 3.5mm is probably the best solution (~$3) since that's pretty much the only way other than optical or hdmi to connect audio to a pc (and what's the point in going the DAC route if you didn't want to go the receiver route anyway :S, though DACs are smaller than receivers since they don't require the amplifier, and the DAC's probably redundant since you probably have a nice sound card if you're looking at $750 speakers)
I think I'm starting to sound like a broken record or something, [repeat: blue sky exo2].
http://www.bhphotovideo.com/bnh/cont...G&A=details&Q=
it comes with a 3.5mm input, making it easy (just a bit easier than rca->3.5mm anyway) to connect to your PC and comes with a subwoofer.