Speaker cables where to begin?

I have a system I am comfortable with so far I believe the cables are the weakest link.

I am currently using transparent audio music wave cables in 15 foot length. Unfortunately, I can't go much shorter possibly 12 feet but that is stretching it.

My system consists of and Accuphase E-408 integrated amp, Transrotor Darkstar TT with JR arm (AKA jelco 750D) benz micro glider 2 Low output cartridge using the Accuphase AD-20 card. Other components are an Accuphase T100 tuner and Arcam DV29 for occasional CD listening.

Everything is run through a Richard gray 600 Pro. Interconnects are older straightwire.

I am not looking to spend a fortune on cables although I was thinking I should use a biwire terminated end for the speakers. I guess the question is what would do the best for my system 1st vs what is most economical improvement? Some day I am sure I will make other upgrades but for now I am looking at speaker cables and possibly power cords and what would do justice to my system.

I did notice an incredible improvement in CD sound quality with the addition of the RG 600. Should I expect an equal improvement from both TT and CD with speaker cables?

Thanks