Step up transformer

I have used pots on transformers and they function electrically as expected.  The 1kΩ value you mention is fine for loading a cartridge directly (across the primary of the SUT).  If the desire is to load the secondary, then a much larger value would be needed.  

As for sonics, a case can be made that the type of resistor also plays a role but these are macro and micro situations.  A pot is quick and easy to change and dial in to find the macro effects of loading on the system.  Once a general range is found, I find moving to discrete resistors often nets a further improvement.

dave

very helpful, thanks so much for reply

@bluetosman ​@intactaudio 

Wow! I am a huge fan of SUTs. And I spend so much time swapping resistors back and forth. It never occurred to me to use a pot to find the exact sweet spot. Of course it would change on any given album or my ears on any certain day. As everything affects how I hear things .But it would be nice to be able to dial it in to find the overall best value before committing to an actual value. That way even if I had to spend the extra money to have the exact value, and high quality resistors custom made. It wouldn't be wasted. I have no idea why I never thought of that. Kudos to both of you! A simple, yet great idea that has never occurred to me until now.

Soundsmith make a phono stage with "continuously variable loading"

So it probably works OK on an SUT too.