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Many tube preamps have a low output impedance that will drive 20K- but not at all frequencies. This limitation is imposed by the output coupling capacitor. The problem is, the bigger you make the cap so it can play bass, the less transparent the cap is. There are several preamps that use output transformers, such as the EAR and Shindo, but the issue that you face then is that transformers also obscure detail and can cause a loss of low frequencies as well.

It *might* be possible to have a technician increase the input impedance of the transistor amplifier in some way. That would widen the doorway of what's possible.

700 miles- you must be in the southeastern part of the country.

I found that Einstein the tube has also low output impedance (50ohm).

Check out the Aesthetix Janus or Calypso.

Mimberman, the Calypso is a good example of what I was talking about. From the balanced outputs, the output impedance is 112 ohms at mid-high frequencies, but rises to 3,900 ohms at 20hz. From single ended outputs, impedance ranges from 2,400 ohms at mid-high frequences to 3,150 at 20hz. Aesthetix will tell you the roll-off is not noticable for most practical purposes, but I did not like the sound of the Calypso driving my McCormack DNA500 which has input impedance of 10K ohms, and I found it to sound somewhat thin compared to the VTL 5.5 and Atma MP-3 (fully loaded). Maybe it was the set-up, or maybe I just like a more harmonically rich presentation, even if it does deviate from "neutral."