OK so I got an email back from Rolf at SL and he suggest staying 100-300ohm.
I would agree with this recommendation. Many say the general rule is 10:1, Amp input impedance:preamp output impedance.
However, in many cases this is not enough. Many manufacturers, like Audio Research, recommend a 20:1 or 30:1 minimum ratio.
The main reason is because most stated preamp output impedance is not a ruler flat measurement, but rises quite a bit in the lower bass regions.
For example, my ARC Ref 5 preamp has an output impedance rating of 600 ohms (balanced). ARC, the preamp manufacturer, recommends an amp load of a minimum 20K input impedance (33:1 ratio).
The reason being that measurements show the output impedance of the Ref 5 rises to 1447 ohms at 20 HZ.
This is common with many tube preamps, the stated output impedance can more than double in the lowest octave.
Another example is the Cary SLP-98 that you mention. The spec sheet states a 800 ohms output impedance.
According to Stereophile measurements, " The output impedance in the midrange and treble was usefully low, at 370 ohms, but this rose at 20Hz to 6.8k ohms, due to the limited size of the output coupling capacitors."
In a nutshell, your manufacturers recommendation of a preamp with an output impedance of 100-300 is a good one for an amp with a 10K input impedance. This will eliminate most tube preamps, but not all.
Cheers,
John