I don't own any Ascend speakers (those Towers look awfully good, though) but just from one who has spent way too much money on this hobby:
You keep saying you know you would be happy with the 170's or the 340's but I say no, you won't. Why? Even though I am sure they are excellent speakers, the problem is that if you bought either of them, the Sierra's would still be THERE in your mind and you will always be wondering "what if?" My opinion is that it is best to spend the extra money and get the Sierras, for your own peace of mind. If you factor in the cost over the time you will be enjoying these speakers instead of wondering what if, the extra cost is not that much.
I've thought the same things you have and all it did was cost me more in the long run plus the time spent without the equipment I really wanted to have because I tried to convince myself that what I was buying was good enough.