Jonathon,
Don't know where you are getting 11kHz from.
The manual says the 6560 driver chip is limited to 15kHz

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The chips voltage supply range is from 6 V up to 40 V so the board can be used (with some
trivial component changes) down to 6 VAC and it might be possible to push it close to 40 VDC
giving some more drive capability for higher inductance motors, but that will require a better
regulated power supply so as not to exceed the 40 V under any circumstances.
The other limiting factor of the chip is it's 15 kHz step clock maximum which limits the step
rate to 3750 steps/sec using full step or 950 steps/sec with micro stepping. With a typical 1.8
step angle or 200 steps per revolution of common hybrid steppers this translates to about 19
r/sec or 1100 rpm and 4.5 r/sec or 280 rpm, respectively.