the hiss could be the electrical differences from the computer and the radio.
you would need something with a 'ground lift' to remove the hiss.
the hiss could also simply be the radio's circuit board being lower quality than needed.
either way.. its expensive to mess around and play.
it would be easier to try a different input.
my radio has a 3.5mm auxilery input.
i can connect an mp3 player without any ground hum or hiss.
but if i connect the laptop to the cord, i get lots of ground hum.
things that have 'ground lift'
preamps are the major thing.
line drivers are also preamps.. but they increase the output voltage.
most soundcards output 2 volts.
the line driver can increase the voltage to as much as 9 volts (adjustable).
since you said its a hiss and not a low hum (or rumble)
i would think the input you were using was junk.
it might be high caliber circuit design.. but obviously the design isnt finished (or high quality enough).
of course, digital input would be the best choice.
but maybe the digital to analog convertor inside the soundcard is better than the digital to analog convertor inside the radio.
headphone enthusiasts go crazy about DAC quality.
they purchase seperate DAC units simply because the DAC is higher quality and sounds clearer.
i looked at the page of your radio, and it doesnt have a digital input.. only a digital output.
the cable should be irrelevant since its not going to help input a signal into a radio and feed the signal to the amplifiers.
the fact of the matter is, line in is line in
you could always try to use one of those FM tuner devices that plug into the antenna, before the actual antenna cable.. but the signal to noise ratio is low, and the overall distortion is probably higher.