I wouldn't worry about any SF tube preamp driving your Classe amp. I've used both a SFL2 and SF Line 3 to drive a Classe CA400 and both were/are fantastic... Kent
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.