If you have some money to spend and you really care about the sound^, you'll get the most bang for your buck by going to a professional music store and buying the kind of gear used in studios. Strangely (or not), you'll be hard put to find singe channel/speaker cables costing more than $50 in any recording studio unless it's in a huge room needing a long run.
I know nothing about your home of course, but when it comes to speakers you basically want them to be heavy and have big woofers. Many studio monitor speakers are powered, with an amp built into the speaker cabinet. In practice, this usually gives better results than a separate amplifier. Mackie Truth monitors will fill an average room and are very accurate for about $1000. At the higher end, Genelec and ADAM are considered the truly desirable names to have. I particularly love the sound (and look) of KRK speakers. If using a subwoofer, 18 inches is fine. More is for reinforcement, not fidelity.
Buy good stands which you can fill with sand to damp vibrations. Speakers should be around the same height as your head in the room's sweet spot, and basically form a triangle with your head as the third point. The bigger the sweet spot, the better the speaker. You must have carpet; a bare wood floor defeats the purpose.
Nor do I know what gear you own, but prefer things which offer 'balanced outputs' - they reject interference and electrical noise far better. If you like records, get a technics SL1200 turntable and an Ortofon needle, for about $1500 altogether. If it's good enough for the world's best DJs, it's good enough for you. CD players from Marantz or Denon are excellent. Ask in the store about your mixing/source selection options, or we'll be here all day :-)
^ First, ask yourself honestly if looks matter. I love the look of studio gear, but that's because of my background as a sound engineer. Not everyone shares my utilitarian sensibility.