WOW! Did I tune in late to this thread or WHAT?!?! HAHA!!!
Actually the big difference is build quality. I know that "Q" word isn't used often to describe these beasts but bear with me. The 931 (at least this 1st version) is not bad by any stretch. The cassete and battery doors close tightly and it has very minimal creaking. It seems to handle the sound and vibration very well. That's really where the difference is. The 975 would rattle a bunch. The cassette doors are quite thin and floppy in comparison. I feel that the 975 does in fact produce more bass, but it does so to it's determent. Although the pretty white 8 inchers are quite robust it seems as if the loudness control on the 975 pushed the signal beyond it's limit and the sound becomes sloppy. The 931 on the other hand is always tight and clear. There is NO lack of bass in the 931, it actually sounds magical!!