The Bent TX103 step-up reduces the impedences seen by the cartridge by a large factor. This reduction depends on the amount of step-up needed. You must set your loading at the preamp at 47K. This will give you an effective load as seen by the cartridge of 470 ohm. The Bent Audio than comes with many resistors to adjust the loading of the step-ups. I found the instructions for this loading to be POOR. You must have Bent give you exact instructions,,,, ? or you can experiment.
The problem now lies in: do you load the pre at 47K ohms, thinking you will keep the step-up, or do you get a pre with easily adjustible input Imp., this will allow you to change easily in the future?.