Where do cables upgrades have the most impact?

yep

Most online cable dealers offer a trial period included with a sale.
So, the next step is to decide on which brand to try.

I would advise introducing one cable at a time when auditioning.

Speaking of brands and models, you will want to try what you never or almost never see used for sale yet what sells new in significant quantity. I am talking real cables and cords not $200 entry level junk.
How would you know ? Not sure, find a way.

To answer your question, mostly on your wallet.

I got a Cardas digital cable for CD transport to tube DAC & it’s made quite an improvement.
I wasn’t sure if it would have much effect.

There is a new level of clarity & accuracy. It does reveal the limitations of some recordings as before there was a veil of ‘bloom’ that had an effect of homogenizing...