Another inexpensive upgrade would be your phono cartridge. Several hundred dollars in this area can improve or certainly change the sonic signature of your system. Nothing wrong with the Oyster but other cartridges can provide greater detail and clarity for not much more money.
All in all don't mess with cables at this point. You have respectable levels already and although cables can have an impact and I believe that they truly do, source components like phono cartridges usually have a more obvious if not profound impact on systems sound especially since it appears that you do not have a digital front end in addition to you analog front end.
I like it and please keep us abreast of your changes!
Sam