i'd get all ascends just for more system synergy, and put the previous speakers on ebay. next, maybe buy more media or signup for netflix? if you're all set on media, just save the cash.
try the htm-200. remember ascend has a 30 day trial so if you find that whatever speaker you try doesn't suit your needs, simply contact ascend and see where you can go next. :)
I'd second BGHD's suggestion to get a pair of 340 for the front and push
your 170s to surround. I'm not sure though, how much $$$ you can
recover from selling your current BOSS surrounds. If a lot, you may have
enough $$$ to get a new receiver (don't forget you can also sell your
current receiver to get more $$). Or, another option will be to push
your BOSS speakers to surround back and all of a sudden you have a
7.1 system :)