i better save the money to change the speaker.
i will not pay 1k power cord if the speaker cannot perform what you need. borrow some cabel from friends to play with first and see what level of your system can be.
playing classical just like car climb the slope. just like a corolla fill in over #100 oil! even though it helps a bit but you cannot run as big cc car.
I think the first problem should be in speaker, so, change the speaker first.
For RCA cable, you can consider audioquest, oyaide or even China made Choseal are good under 1K.