Sorry not to get back sooner (very busy with my day job). I found that if I turn the bass way down on my computer speakers, then what I hear is flatter, and I guess that's the "true" pitch. When I turn the bass way up, it sounds at least a quarter step sharp. And with headphones, it's flatter (apparently the "true" pitch). Also, I had my computer's subwoofer almost against the wall, and when I moved it away (room treatment, as batsbrew said) the tone got flatter, so I guess the bass was vibrating the wall at a slightly sharper pitch.
Now I understand better why the bass pitch varies. But what's weird is that I only notice this happening with my Sonar piece, not with tunes I hear on YouTube. Maybe it's my choice of Cakewalk's SI-Bass
Guitar - the default sound, which has so little treble in it. I'll experiment with other SI-Bass
Guitar sounds and see if I have less trouble with them.