like how you posted this thread, it's a good question- as the description of advancement per speaker model is not exactly clearly defined. Perhaps this is on purpose as the speakers as a whole are fantastic- and some may not be made to 'replace' other models but are possibly complimentary in sound but for different use.. im thinking of this new tower and it's probable fullrange abilities- something I personally do not find important since I am a fan of pairing with a dedicated sub-- now I'm sure the tower offers more than just that one feature, as it will blow away the 170's in many other aspects- no doubt.
Back to the original question and request for a beefier description would be nice. After some digging I bumped into this thread back on this forum actually:
http://forum.ascendacoustics.com/sho...2&postcount=17
For starters I am curious as to why the spectral decay image is not the same as what's on the main site, but that's not too big of a deal; but what the post states is the major advancements of the 340 vs the 170..to go back to the OP I think some of the major advancements of the sierra over the 170 are much more detail heard, great FR from off-axis angles, and the ability to run without a sub if needed- they go pretty low for many people's tastes.
My response isn't meant to actually answer any questions for you, but just agree that more definition as to what each product has on eachother would be nice; but perhaps that's bad business as it plays down previous products..I have no idea.. what would be good is a simple guide to select speakers based on a few simple descriptors (like no sub used, large room, mostly music) and it'd recommend the sierras as a stereo pair for purchase; or (need a sub, want 5 channel, not much room for rear-surrounds) and it'd pop up rythmik sub, and some htm's for the rear-- explaining that they can be placed on the wall, directly mounted. its just an idea.