I haven’t followed all of this thread, but is the OP stating that he swapped out ONLY Internet cabling to his device, or added a component to the device(s) too?
I do software for a living, and understand the TCP and IP protocols used to move byte sequences over the Internet. TCP/IP underpins most aspects of Internet traffic: email, web, ftp (showing my age!). When the sender (e.g. Spotify) transfers a ’file’, which is really just a byte sequence, to a receiver (your music box), it adds checksums to ensure any errors in xmission are noticed by the receiver, and fresh copy sent. In essence, TCP ensures that what the receiver gets is what the sender sent.Now, if music streaming is what I think it is, byte sequences of data representing sound files, then there is no way a cable by itself can ’improve’ on the data sent from S to R. Yes, it may be faster, or be higher quality and require fewer re-transmits, but at the end of the day, an Internet cable’s job is to xfer data across its two endpoints, which is likely one small ’hop’ in the larger path from sender to receiver. The cable doesn’t ’know’ the signals its moving are sound files, its just electrical signal representing 0 and 1, each a ’bit’ and we have 8 bits to the byte. Only improved PROCESSING (better DAC etc) of the incoming byte stream can possibly improve sound quality. The D in DAC is a given, it's what TCP/IP does. It's the A in DAC that counts. And cabling alone is firmly in the D side of things.
That said, I'm off to polish my two grand power cords, they do make a difference, honest ;)