Murph,

You can buy shielded and unshielded cat 5. If your 500 is anything like my 600, I don't recommend the use of cat 5 cable. My 600 is extremely sensitive to noise and the plastic connectors on cat 5 don't provide shielding at that critical termination end. You may be fine of course depending on what your electro-magnetic environment is like but I see it as an unecessary risk.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated