Hattor Audio is now offering a Reference Preamplifier using autoformer attenuation (link) instead of resistors. Think Slagle, EM/IA, and Pal Nagy's icOn.
I have successfully used their flagship, The Big Preamplifier, in passive mode to provide volume control for my system. The Big Preamp has both RCA and XLR inputs and outputs, an outboard power supply, a large display, and remote control of power, inputs, display brightness, mute, passive/active operation, adjustable gain, and volume for each channel (i.e., allowing control of balance).
The new autoformer based reference preamp is priced reasonably like all Hattor gear, and offers four autoformers for the balanced version. Either copper or silver autoformers are available, but the silver version more or less doubles the price.
Anyone here tried one yet?