I don't think the cable has to be a special one. Before I sold my GT6 I used a standard RCA cable, that I borrowed from my hi-fi. The thing about the sound issue is that you have to press the "Output Select" button (I think it's named like that) and then select the "P.A. / Studio" setting (again, can't remember the exact name). This output option works on the SPDIF out as well, and makes the unit sound a lot better, than the other options available.
Although the manual states your presets will sound the same, whether you plug into a PA or in front of an amp, using the corresponding output-setting is not true. You still need to tweak (and tweak and tweak...) your GT6 to get it sounding right. ...but I found that using the GT6 directly to the PA with the right setting, it sounded the best (for my style, Metal)... but in the end, I sold it because I got myself a POD2, and it just sounds sooooo much better, in my oppinion.
Dime...