The rule of thumb is the input impedance of the amp should be at least 10 times the output impedance of the preamp!... at the frequency within the audible range for which the preamp’s output impedance is highest. Which in the case of preamps having capacitively coupled outputs (such as the majority of tube preamps) will usually be at 20 Hz, and the output impedance at that frequency will often be far higher than the specified output impedance (which is usually based on a mid-range frequency such as 1 kHz).
That doesn’t mean that there will necessarily be a problem if the guideline is not met. It depends on how the output impedance **varies** as a function of frequency. What it means is that there **won’t** be an impedance compatibility problem if the guideline **is** met.
In this case, however, the Coincident Statement Line Stage has transformer coupled outputs, rather than capacitor coupled outputs. My suspicion, therefore, is that at all frequencies within the audible range the output impedance won’t be a great deal more than the specified 500 ohms. So I suspect that a 15K load would be fine. You may want to check with Israel Blume of Coincident to be sure.
Regards,
-- Al