how about this: changing ANYTHNG can have an impact on a system's sound. there is a lot to say about the synergy between components. like certain power amps work better with some speakers that others. this makes saying that one wire is more important than another in a certain position difficult.
if I were you and trying to improve your situation, I would work backwards from the endpoint, upgrading cables in that manner. your system may need 100 hours when you change something. and you should do a lot of listening with the new part, and then go back to the old one and see if you can hear anything, different isn't always better. everything is returnable - or should be. unless you're one of the people - who I greatly admire - that will buy used stuff and then resell it if they don't like it
the other thing I would recommend is linear power supplies everywhere, again working backward from the end point and going all the way to the modem.
it's a great hobby - and Happy Thanksgiving to you!