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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.

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."