After I posted this I vaguely remembered a Stereophile review with measurements, so I have the data I was looking for. The Brilliance Control plays a huge role in what the amp sees. I’m currently running the Brilliance Control at ~1:00 position, which according to the Stereophile article should put minimum impedance somewhere between 2 and 4 Ohms, which should be fine for most good amps.
A lot of amplifiers understandably struggle driving impedance below 2 Ohms. I know Sound Lab impedance drops pretty low above 10kHz, but I can’t remember seeing a curve. Where can I find an impedance vs. frequency? If an amp says it can’t be used below 2 Ohms is that a show stopper given the relatively low energy in high frequency music?