FirstWatt SIT-2 Manual page 5

The distortion performance of the SIT-2 into both 8 and 4 ohm loads is shown here:
In both curves we see a second harmonic character culminating in a very soft overload, which
gives the amplifier more apparent power than its distortion ratings suggest.
The SIT-1 and SIT-2 amplifiers can be driven in "direct mode" where the source components
looks directly at the Gate of the SIT without other active components in between. This is
desirable, but in this case the input impedance is 10 Kohms and plus the capacitance of the
Gate which is greater than the 1,000 pF of the IHF standard. If you have a preamp or source
component with an output impedance of 1,000 ohms or less, you should not have a problem,
but with "passive preamps" and some tube designs which have very high output impedances,
this limits the bandwidth and could be a performance issue.