This may sound counterintuitive, but we have found the best cable upgrade path is:
Power --> Speaker --> ICs --> Digital
In case you are interested, Finley Audio (I am one of the owners) will be partnered up with a Boulder 866 this year at Axpona 2023. We will be using our Cirrus family of cabling to connect the whole system if you want to stop by.
If you cannot make it, please feel free to call or reach out to us. We would be happy to discuss your specific system and help you in any way we can. We also have a trial period for our products to make sure they synergize well with your system.
Good luck in your search and happy listening!