I hate to add a new layer of discussion for this topic, but I'm going to anyway!
All this talk about sounds is MASSIVELY dependant on where and how the boombox in question is going to be used.
If someone's going to use it indoors at low-medium volume nearly any box of reasonable quality will have generally great sound. Some may
need some better positioning for maximum quality, but you can get get great results from all but the cheapest of cheapies most of the time.
If you plan on using it out side in a stationary position then you need more power for the sound to be able to carry at a decent volume and clarity.
Lots of boomboxes weren't made to go above 7 for any amount of quality sound, and sometimes that's not loud enough for them to do the job.
That said, the TRC-931, S90, M90 and GF-535/9000 can do all these admirably.
If you're planning on carrying your boombox however, AND you want bass while carrying, you'll probably find the 931 and M90 will be the only
2 up to the task (of these 4). Simply because the volume level carrying is probably the highest you'll ever have it. I'm constantly surprised at how loud a boombox
has to go to get good sound while walking around with one. Especially when I get indoors and have to cut the volume by about 50% to be able to hear absolutely anything else. To get good bass and overall sound quality at volume this is the ultimate test, especially busy areas with traffic and such. I've got a lot of boxes that I can carry but only when I get the settings perfectly balanced, and this is nearly always at the expense of the bass so that the volume is still reasonable.
The S90 although a supremely magical box indoors is not up to the task of being carried and played at high volume. The M90 is stellar with both. I'd take a TRC-931 over a GF-9000 any day of the week for carrying purposes too.
It's all about how you're going to use them though, that's the question you need to ask before deciding which box is going to be up to the task I reckon. And the plainly obvious solution is to have all 4. S90 for inside listening, Gf-9000 for cranking outside, M90 for powering an outdoor party and your TRC-931 for walking around with. Problem solved!
Rock On.