No doubt the amp/speaker match is very important for quality sound. Rather than suggesting my favorite speaker like most posters do, I'll make a comment.
Due to the parameters you mention in your post, a higher sensitivity speaker will be better.
Classical can be some of the most dynamic music available. I seriously doubt your current amp/speaker match can deliver that type of dynamic range. Compounding this issue is that the room is fairly large.
A speaker with a sensitivity of ~95db or higher on the 20 watt tube amp will take you to the concert. To get that type of dynamic range on the Polks would require at least 100wpc.
Due to the parameters you mention in your post, a higher sensitivity speaker will be better.
Classical can be some of the most dynamic music available. I seriously doubt your current amp/speaker match can deliver that type of dynamic range. Compounding this issue is that the room is fairly large.
A speaker with a sensitivity of ~95db or higher on the 20 watt tube amp will take you to the concert. To get that type of dynamic range on the Polks would require at least 100wpc.