First and foremost, if you can make it down to Ascend for an in-person listen, you should absolutely do so. Nothing beats hearing these in person. Plus Dina is just fantastic.

Now, the difference between the 170s and the Sierra LX is night and day. The 170 is a fine speaker for its price. The LX is a truly exceptional speaker, period—not just for the price. Obviously the LX costs quite a bit more, but the difference between the two is pretty striking.

You might also consider the Sierra1 V2 and a sub. I would definitely audition the LX vs. the S1V2 at Ascend and if you find the difference not that big (other than bass response, which is definitely better with the LX), you might take the money you save and use that to buy a sub. I find the LXs go deep enough for music (and I'm a bassist, so I'm pretty demanding on bass response), but for HT you really want to get even lower than the LXs go, which means you want a sub.

Personally, I'm still waiting on the S1 to S1 V2 upgrade kits to become available so I can hear the LX vs the S1V2 in person...