Gotta love Google and Stereophile...
From the Stereophile Test Measurements of their review of the
SP-16L preamp and VS110 amplifier:
43k ohms is an input impedance spec that would indicate a good match with a component that has a high output impedance. Therefore, bass response (and treble response) would be extended and sound very good when mating the MHDT Labs DAC with the ARC SP-16L preamp.
From the Stereophile Test Measurements of their review of the
SP-16L preamp and VS110 amplifier:
...the input impedance [of the SP-16L] was a moderately high 43k ohms over most of the band, this dropping to a still high 38k ohms at.
20kHz.
43k ohms is an input impedance spec that would indicate a good match with a component that has a high output impedance. Therefore, bass response (and treble response) would be extended and sound very good when mating the MHDT Labs DAC with the ARC SP-16L preamp.

