I suggest looking for a preamp with an output impedance of 350 ohms or less (lower the better, IMO), and with a differential balanced design. Pass Labs work best with lower output impedances, and Pass Labs suggests running them balanced.
Check the output impedances of recommended preamps, and do a search of available Stereophile reviews. The Measurements section of those reviews will go into detail about the output impedance specifications, and the typical amplifier that will work best with each preamp.
Check the output impedances of recommended preamps, and do a search of available Stereophile reviews. The Measurements section of those reviews will go into detail about the output impedance specifications, and the typical amplifier that will work best with each preamp.