Help Finding my system bright...

Macd, given that your other components are not thin-sounding, I would suspect a computer-driven digital source (if that's the case) with Windows or Mac OS doing their usual tantrums below the table, like downsampling by default all music files to a lower-resolution format. It's been known to happen to many a computer-savvy user.
Just a hunch...

How would I know whether it is down sampling or not?

I suggest that you ask this question at a PC sound forum. There are some very knowledgeable guys there.
I have read that Vista and Itunes downsample by default. Due to continuous virus atacks and technical problems I am no longer playing in Microsoft's sand box.
I have the latest Imac with Itunes and CD copies sound slightly worse than the original CD.
Otherwise, it is a much better, ergonomic, trouble-free platform, a joy to use. I wish I made the switch earlier.
Good luck