I anticipate a great deal of HP-blaming in my near future.
My reasons for not thinking it picks up on "line in" are chiefly that when I use the Recording option on the volume control, it only picks up on the "mic" fader. I've also mucked about a bit in recording studios in the past, and, well, this sounds a lot like what I've experienced when I plugged a line into a mic input...I have to put the "mic" fader ALL THE WAY down and the stereo volume NEARLY all the way down to get a resonable (and very, very noisy) non-clipping level...my experience with line input is even if you run out of a headphone jack, putting the volume "kinda" low (30-40%, as compared to, like 2% in this situation) it can work, while this really, really behaves like I'm plugging a line into a mic jack.
I'm already asking for suggestions re. good external USB sound cards and etc. in anticipation of this being an insoluable problem...thanks to someone else I now know how (theoretically) to build a kind of "de-amp," but building a de-amp to foil a pre-amp really seems like it's redundant and a very bad idea, sound-wise.
Still, any and all suggestions are welcome...