This sounds very strange to me, especially since in my experience the 3A Sigs are not bright sounding speakers at all. What are you using for a front end and cables/interconnects? I'm almost wondering if there's something wrong with one of the components or the speakers. I'd try another amp just to see what happens and go from there, but something just doesn't seem right here.
Just out of curiosity, what speakers were you using before?
Just out of curiosity, what speakers were you using before?