If you can't hear a difference in comparison testing where there really should be one as referenced by other sources or reviews, figure out why, you obviously have a resolution problem that is limiting the detail level. To say as all too many "audiophiles" are prone to doing, that "they can't hear any difference so it's a waste of money" is the purest definition of ignorance in the audiophile dictionary.
Whether to focus more money into cables or components, and which components, is the the oldest argument in the industry next to does or how much difference do cables make. In the most modest arrays of equipment, especially where you know you have one of the price point over performers, going beyond what seems to make sense in relationship to what you've spent makes sense if you can get into resolution, detail, and music enjoyment that doesn't seem possible. I always kept an entry to higher end products and spent as much on cable as the equipment was, if not more. The performance that people leave off the table in entry level systems is unbelievable until you hear it. I used to show people two levels of performance between a Rega Jupiter and the planet, both excellent performers at their price points in their day, $900 versus $2000. Comparing them with an entry grade neutral cable then the top tier interconnect by the same manufacturer radically transformed the difference you could hear between the two. By that same exact logic you can then do the same thing between the $2000 CD and a great $5000 one and there will not be all that much difference as there is with a cable set that lets you hear it. Put the better cable in and the differences are profoundly greater.
In order of the flow of the signal from source to speakers the most important is the cd to preamp. That said if you have a weak speaker cable in you won't hear all the detail that the interconnect upgrade does. Speaker cable first even though it might sound bad because it nay be revealing bad interconnect coloration.
