I have followed this thread with great interest. I thought I would chime in as this is an exceedingly timely question for me.
I would note, Up until about two weeks ago I had my speakers and components on different walls requiring 60 feet of speaker cable. A couple of the (non-obnoxious) cable threads made me reconsider my physical set up. Now everything is on one wall and my speaker cable is not 2 meters. Same mediocre brand but basically 54ish feet shorter. I think there was a decent change in the quality of the music coming from my speakers. I think that helped.
At the same time, prior to this change I engaged with an online retailer with a very large cable library (again, I apologize not sure if we can or cannot name and I have no interest in running a foul of the rules.
I have received three digital cables to try between my DAC and streamer. We started here because speaker cables at 60 feet would have been ridiculous. However, I now wonder - should I pause on the three digital cables and pivot to the speaker cables - again, directly to your point - what is my biggest bang for my buck?
I don't know. I do know, I am not prepared to go down the power cable path just yet. I want to complete my interconnects and speaker cables before I go there. And even before that - I want to have a dedicated plug and line run for my outlet.
What I might offer - someone had suggested a change from Ethernet to fiber when plugging into my streamer. The three components were: 10Gtek Fiber Patch Cable, 10Gtek 1.25G SFP Transceiver 1000Base-LX, and TP-Link MC220L. All in for those three items was less than $80. I felt a significant difference in the sound quality of my streaming.
I look forward to the comments from others. A number of people who have already commented have been very influential in helping me think through this cable/audio journey and I am appreciative.