Was just poking around TentLabs site (hadn't been there for awhile) and noticed that the tube output stage he uses for his DIY CD player claims an input impedance of sub 1 ohm.
http://www.tentlabs.com/Components/page4/page4.html
The IV converter uses an E88CC valve as transimpedance amplifier. It fully operates in the current domain, and is a new and novel design. The advantage over existing tube based output stage is the low input impedance (typically well below 1 ohm) which reduces the distortion generated in the DAC chips compared to the situation when these are loaded with say 100 ohm, which seems common practice these days. After the E88CC stage, a passive filter follows. The actual IV conversion takes place using an Audio Note tantalum resistor, signal coupling is achieved using Audio Note copper foil paper-in-oil capacitors. 2 pairs of outputs are available, a straight output with a moderate 2.5 k-ohm output impedance, and a buffered one with output impedance well below 200 ohms. The IV stage is fed by a custom made toroidal transformer, the well regarded 6X4 takes care of high voltage rectification.