" Would cables costing $300-500 for example have any improvement over my $23 ones?"
Depends. Best to try cables that you can return if you don’t like them.
Given the length of cables you need (I suspect you are talking 25 foot speaker cables) I would suggest you rather buy a good power cord for that Luxman and leave all else as is for now.
A pair of worthwhile 25 foot speaker cables that will make a noticeable difference will probably exceed your budget. Somebody else may have a recommendation that falls within your budget, but I don’t.