The AU-300LC is versatile enough
As long as you don't need more output, you don't need a different SUT.

While in standard form, it's a perfect match for the DL-103 (higher impedance than 103R).
You can tune it by loading it's secondaries with resistors.

110kOhm when using the DL-301II.
33kOhm when using the DL-103R. (don't worry about signal loss, the 103R has more output than the manual states)