Two things for me when thinking about this. 1. Headroom, going from the PS Audio amp that has tons of headroom to something much much smaller that may not have that extra in the gas tank.
2. Damping factor, going to small tube class A amp you might be lacking there compared to the PS Audio amp probably has DF in spades.
But I think overall you should be fine. Your ears will tell you if the SF speakers will get enough to sing.