They work excellently with both tubed and passive preamps. They were designed for use with Steve's passives, but have a certain magic combined with tubes, actually.
Hello Ehoehn -
Most of my amps have a 100Kohm (high) input impedance and are easy to drive with any type of preamp. The only amplifiers I produce with a 10Kohm (low) input impedance are the DNA-500 and the SMc Audio-upgraded amps that include my new balanced input circuit or those that have been converted to monoblocks. I now use a transformer for input balancing and phase-splitting, and this results in a 10K input impedance. Most active preamps will drive this without any problem, and many transformer-based passive systems (like the Bent Audio NOH) also do a fine job. However, passive units that use conventional volume potentiometers or resistive-ladder attenuators (without active output buffering) and a few tube preamps with unusually high source impedance (over 1000 ohms) are not likely to give the best match.