Pass XA30.5 enough power for Wilson Sophia 2?

That is quite a line-up of great preamps. I did once use the Audible Illusions 3A with the Aleph 3, which I think was 10kohm input impedance, and the AI3 was realtively high outut impedance, and I did not notice any problem driving the load. I would say that 95% of preamps should be OK with the Pass, it is an issue, but is not likely a problem with most, if not all, preamps - certainly not ideal for those seeking a passive path to preamplication.

My understanding is that a 600 ohms preamp impedance output (like the Ref 3) requires an amp with 60k input impedance.

Tvad is correct.

The point to using a ratio that approaches 100:1, or that is much larger than 10:1, would apply to the situation where the only output impedance specification that is available is a nominal rating, that presumably applies to mid-frequencies. In which case using a high ratio would allow for the rise in output impedance that commonly occurs at very low frequencies, especially in tube preamps, due to the output coupling capacitor.

In this case, worst case output impedance data for the Ref3 across the entire frequency range is available in JA's Stereophile measurements. The worst case, which as can be expected occurs at 20Hz, is 1437 ohms in balanced mode. That is well under one-tenth of the input impedance of the Pass .5 amps, and therefore should not be a problem.

Also, in situations where this issue may be a problem I would expect the symptom to be attenuation of the deepest bass, not muddy bass.

Regards,
-- Al