I am not familiar with the Classe pre, but do own a Line 1. The SF preamps in stock form can best be described as very neutral, with none of the warmth typically associated with tube equipment. With an appropriate NOS tube change, however, these pieces can come alive. At present prices, they are also a wonderful value for the quality you get. Being unfamiliar with your Proacs and your front end, I cannot say how the stock combination would sound out.
Classe vs. Sonic Frontiers
I currently own a Classe 5 Pre and power combo. I am wondering if changing the pre to a Sonic Frontiers Line 1 or greater would be a huge improvement. Also would be interested in the phono 1 to replace the internal phono in the Classe. If anyone has experience using Sonic and Classe let me know what your findings are. I am powering Proac speakers with this setup.