Does the length of an interconnect degrade the sou

The short answer, yes. this is a lot of cable for a DVD player.

It can, yes, however it depends on the shielding of the cables and how much external EMI and RFI is infiltrating the area near the cables. Other factors also apply.

If you have truly balanced (XLR) in's and out's, 30 feet is nothing!! If you are using unbalanced RCA's you will probably have some signal quality loss.

Fatparrot makes the key point here: balanced interconnects were specifically designed for use in recording situations, where the length of the cable could be significant. For short runs under 2 meters or so, unbalanced interconnects are usually fine, but for a 30-foot run, I second Fatparrot's recommendation to use balanced IC's.

The only real difference in susceptibility to noise pickup and that depends on the RF/EMI environment which is quite noisy in studios. I run 30foot interconnects from preamp to amp and there's no consistent difference between balanced and unbalanced with regard to noise or signal attenuation.

If the source can drive the line and the load for the length, it's fine.