I've used a Yamaha C-80 preamp previously with such good results that I ended up retiring all 3 of my Little-dot units. I never could find specifications for the the headphone amplifier in the C-80. The C-4 however, does list it's headphone specs in the manual.
For "Output Level / Impedance" it specifies "18V / 330 ohms".
I don't know if 330 ohms is referring to it's output impedance, or the headphone impedance at which it can output 18V. I also don't have much point of reference to know if 18V is a lot or a little. I'm trying to get a feel for if it prefers higher or lower impedance headphones. It does sounds quite good with my 250ohm DT990-Pros though