I can't say exactly, since I'm no engineer. But I will say that if you just want to record guitar/bass you might be making it harder on yourself by trying to track down noise issues in your cables.
The problem is likely not your soundcard. An old soundblaster can work fine, IF you use the line in, AND going through a mixer or mic-pre. Better yet, as others have said, to use an USB interface and bypass the card completely. The DI from your amp is meant to match impedance with a mic input (a proper one like on a mixer). It's essentially a line level signal, which means it needs to be (properly) amplified. You can use the mic input on the card, but it will likely sound horrible and distort. Have you tried the DI on any other source to see if it works right - shouldn't be any noise at all.
If no one has ever opened your XLR adapter cable, it's probably fine.
The problem likely has to do with the way in which your connecting, using so many adapters and such (lots of impedance mismatching, etc..., adding noise). Get yourself a budget USB interface, or at least a mixer that can plug into the line in on the PC card (usually stereo RCA to 1/8 input stereo plug). You'll have to figure out how to monitor and deal with latency issues, regardless of how your record.