If you can find one in good shape, the Markof PPA-2 based on a battery-driven Marshall Leach design is a contender. Back in the day it was considered one of the closest contenders to the Counterpoint SA-2 pre-preamp, which was acknowledged the finest. I had both, and continue to use the PPA-2. The SA-2 had wonderful, more dimensional tube sound but was a bear to get around humming.
