Looking to upgrade the pre-amp as the Avalon's are either showing up the pre-amp and or it's just not a good fit (it worked well with my Thiel 6's).What's problem with the sound? What makes you think problem is pre? Avalon sounds very different than Thiel.
I've had SF Power 3SE+ and SF Line3SE+ (mod to the MAX by PCX) and many more SF pre so VERY familiar with their sound.