From my post at the Ascend forum:
Based on my listening at Curtis's house last night, there is a definite difference between the speakers.
If they were photographs, the 340 would be slightly out of focus compared to the Sierras, as if there were a very fine gauze between me and the speaker.
Given my experience with the 340, this is insane since the 340s are simply excellent.
The bass on the Sierra is better, not merely because it goes lower, but it is tighter and punchier. Again, given the strengths of the 340, it was an odd experience.
I think the cabinet is making a significant difference. I noticed a significant improvement, for example, in the sound in my car when I sound deadened the doors. I think the cabinet on the Sierra has much of the same effect. I'm pretty ignorant about the tech side of sound, however.