I ain’t frowning lol … connect what works for you ;_)
Usually if you connect something that has stereo RCA connections (i.e. your S2) which sit closely together to another device (i.e. Denon 4311) that has stereo RCA connections that sit closely together, it works having a regular stereo RCA cable.
Like this one: http://static.bax-shop.nl/components/com_virtuemart/shop_image/product/Procab_ref800_5.jpg
But … when you switch to KRK’s which are active speakers with the amplifiers built in, they will have ONE RCA per speaker. This means you will have to have one RCA to RCA cable going to the left KRK and another one to the right. You can take a knife to your stereo cable, but changes are you will cut and breach the outer shell.
So, if you get seperate cables to begin with you can still use them for your current hook-up, but also for when you switch to KRKs or any other kind of active monitors. They ARE harder to find though and might be relatively more expensive. (I usually make them myself).
For what you are trying to do now, stereo RCA is fine though. They are not too expensive, the one in the link I included is about 10 euro for 3 meters (9ft) and is a decent cable for the job. If you ever move to KRKs or similar or start playing out, you could get yourself some balanced Jack/Jack or Jack/XLR cables then.
Greetinx,
C.