First Q: why use a stepup tranny at all? The GPCH has 60db of gain at its highest (unbalanced) setting, which should be adequate for a 103. You would experiment with its 100, 500 and 1K impedance settings to see which gives you the flattest response.
If you must use the K&K's for some reason, leave them at 18db of gain and set the GPCH for its lowest gain setting (42db on the unbalanced inputs). That will give you 61db of phono gain, which happens to be "perfect" for a 0.3mv cartridge, according to the
KAB Preamp Gain Computer. :-)
Leave the GPCH's input impedance at 47K, that's what the K&K was designed for. The other impedance settings would be far too low when using a SUT.
This will leave your cartridge seeing an impedance of 47K/64 = 734 ohms. That may sound okay, given the 103's fairly high internal impedance of 40 (or 45?) ohms, but it is somewhat system dependent.
If this setup's impedance is off it's likely to be too high, which would over-emphasize the HF's. The good news is, too-high impedance can be adjusted. If it were too low you'd be out of luck and would have to change component(s).
Once the cartridge and trannies have settled in (give them 100+ hours), if the HF's seem excessive the fix is to insert resistors between the K&K and the GPCH. The optimum value would be very system dependent. Fine tuning impedance in a LOMC/SUT combination is very finicky, and in-system experimentation is the only way to do it. The resistors which yield the flattest response in your system would not do so in my system, or anyone else's. This is by far the most difficult and time-consuming aspect of using SUT's, and those who ignore it can never be sure they're getting the best performance.
