The Sierra-1's seem to come from a good lineage (Usher) so that speaks well for them. Personally I haven't had ears on them. I would see what Ascend Acoustics return policy is. I believe they have an in home trial period.
I don't know if you would be happy with them as FL/FR in an HT setup vs the DefTechs. They will most likely not have the punch.
I believe they are rear ported so that may pose a problem for you.
While the Sierras aren't real efficient you should be able to drive them with your receiver just fine given a reasonable seating distance and room dimensions (which are?).
Don't be shocked if your Adcom doesn't buy you much in the way of SQ. But give it a whirl anyway.
There you go then, an opinion