That is very low for a tube amp but the Lamm L2 Reference is 130 Ohms. The McIntosh C1000T is about 220 Ohms. Both are about $17,000.
Our MP-1 has a low output impedance- it has a patented direct-coupled output and can drive 32 ohm headphones quite well.
It was intended to support the 600 ohm balanced line standard. You may already know this, but the low impedance standard was created to eliminate interconnect cable problems and differences, and to allow long connections without degradation. That's how the record labels like RCA and Mercury were able to produce recordings that are still revered today- 50 years later- while running as much as 150 feet between the microphones and the input of the tape recorder.
Our original MP-1 was the world's first balanced line all tube preamp for home audio, introduced in 1989. Traditionally tubes were coupled to low impedance balanced lines using an output transformer, but we were making OTLs already, so it seemed the logical thing to do. With that it appears that we introduced high end audio to balanced line operation. Sorry to sound like an ad but that's the history.
There are several that should work. The Sonic Frontier preamps all have low output impedance, with the Line 3 I believe dipping just below 100 ohms. Ralph's Atmasphere preamps should be able to drive just about anything. VTL 5.5 is also pretty low, and I am sure there are others. My guess is you don't absolutely need to go as low as 100 ohms, but what you must watch out for are the preamps that have low output impedance in the midband frequencies, then a dramatic rise in output impedance in the bass frequencies - these will roll off the bass and will not sound as good. I had similar matching issues with my McCormack DNA 500 that has 10K ohm input impedance.
Mitch2 that's what I'm looking for, to match with ss 20Kohm input impedance and not detroy the bass frequency.
I'd like to have tube preamp but so far there are not many canditates and also is not easy to know the output impedance in all frequencies.
Atmasphere is looking good but no dealer in over 700miles.
I also saw VTL 6.5 and 7.5 with low output impedance but I don't like their sound.
VAC is also a canditate.
Sonic frontiers are out since I'm looking for existing company product.
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.
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."