09-27-08: BlindjimMy experience trying several tube preamps with Pass Labs solid state amps tells me the ratio rule-of-thumb is a only useful as a general guideline, and may not apply in the real world.
...one of the two mainstays, either Mr. Conrad, or Mr. Johnson said a ratio of 10:1 is sufficient for preamp to amp matching. So if the preamp in question has an output imp of say 1000 ohms total, then the amps input imp needs to be 10,000 ohms or better
Recently, I tried an ARC REf 3 preamp (600 ohms) with Pass Labs XA-60.5 (30kOhms)...a ratio of 50:1. The match was not ideal. Not bad, but not ideal.
If one reads Stereopile's measurements section of their preamp reviews, one learns that output impedance changes with load. For example, the Audio Research Reference 3 has an output impedance spec of 600 ohms balanced, but in J. Atkinson's test of the preamp it's output impedance rose to 1437 ohms at 20Hz. This would indicate an early bass roll-off when used with some solid state amplifiers, and indeed when I tried the Ref3 in my system with the Pass Labs amps the bass was lacking some depth and control.
Atkinson wrote about the Reference 3:
"The output impedance was also to spec., at 635 ohms balanced and 326 ohms unbalanced in the treble and midrange, but rose to 1437 ohms and 625 ohms, respectively, at 20Hz."
So, the moral of the story is that output impedance specifications are only part of the tale. Test measurements are another part of the tale. Listening to the actual combination completes the tale.
Regarding VAC amps, it may be true that Kevin runs some of them on Thiel 2.6 speakers, but my experience with the 160wpc VAC Musicbloc monoblocks on Silverline Sonata III (5 ohm minimum impedance) was that the VAC amps produced an unbalanced sound lacking bass depth and definition, leading to an overall "tipped-up" presentation due to the lower power output into the lower impedance frequency region (bass) relative to the higher impedance frequency regions.
I absolutely agree that total WPC is not the most important criteria. However, the impedance issue is crucial, IMO.