Actually 600 ohm is an old tube-era standard for balanced lines. Most line-level unbalanced audio gear (sometimes called -10dB or consumer) is built with a low output impedance (often 1K or less, as low as 100 ohms) and high input impedance (usually 47K, can be 10K to 1Meg.) RCA cables invariably sound identical under these conditions, as the output impedance is low enough to not allow cable capacitance to affect the frequency response.
For RCA interconnects, look for solid plugs and reasonably-rugged construction to minimize faulty connections or broken inner conductors from moving equipment around or what have you.