From what I understand, the VSS signal is for the speed volume control. As the vehicle speed changes, the amp receives that signal and adjusts volume (or gain or something like that).
As a side note, do all '09 Legacy's with the the HK system have SVC, or is it only certain ones? I'm positive my car does not have it, and I could find no mention of it in the manual, yet everybody keeps on mentioning it. I guess it doesn't really matter at this point since I went aftermarket, but I am curious.