I would think you'd at the very least hear a very big improvement in bass weight and extension since this is an area of strength for the Ushers and not so much for the 805s. That said, I don't think the Ushers will open up and sound their best with 100wpc as many reviewers have stated they really need/appreciate more power to show what they can do.
That aside, I would think something like Silverline 17.5s could offer a significant upgrade and work fine with your amp. Best of luck.
That aside, I would think something like Silverline 17.5s could offer a significant upgrade and work fine with your amp. Best of luck.