There is a bit of confusion here, let me try to help.
SPDIF is your digital output, before the digital-to-analog conversion of your DAC.
Typically it works this way:
1. Computer puts out digital audio on USB.
2. USB DAC reads the digital audio, does the D-to-A conversion, and outputs stereo analog on RCA stereo cable pair
3. AMP takes in the stereo analog signal and drives your headphones or speakers
SPDIF (which can run over a single coax cable, with 75 ohm RCA connectors, or over optical TOSLink) would not be used in this situation. If you use certain sound cards in your computer, you might have SPDIF out, and then you would put that into the SPIDF input on your DAC, and then proceed as above: analog RCA stereo pair in to your amp.
If your amp has a SPDIF in, then it must have its own DAC inside. This is typical of some Hi Fi Receivers, but generally not amps. Doesn't matter, don't use it. You have an outboard DAC, that is what you use.
If your DAC really has SPDIF outputs, as opposed to inputs, that is pretty rare. Some do (like the Matrix) -- but then these just pass thru the digital signal for use with some other DAC downstream.
So don't overthink and do this the standard way. Just get a pair of RCA stereo analog cables and plug the DAC in to the amp, the way it is supposed to be done.
Radio Shack cables will work fine. If you want a step up (might not be audible in your setup, if it is really cheap-o, as you say), then I agree with Uncle Erik that Blue Jeans Cable is always your safest bet. If you buy Monster or some other high-end brand at an big box store you are very unlikely to hear any difference from Radio Shack for short cable runs. Keep the cable runs short, and away from power cords. If they have to cross power cords, do so at right angles, not parallel.
