I tried a Ref3 with XA-60.5, which have input impedance a bit higher than 20k ohms. The match was OK, but overall the sound was flat and uninspiring. I was unimpressed with the Ref3/XA-60.5 combo.
IMO, you want a preamp with output impedance under 200 ohms to mate with the 20k ohm Pass X-1000.
IMO, you want a preamp with output impedance under 200 ohms to mate with the 20k ohm Pass X-1000.