Ha! I think this is the first "tone test" I've ever gotten right. Still, they are all pretty close, but 3 to me seems like it would be a nice fit for a lot of mixes.
To be fair the differences aren't exactly huge as they're all just variations on the same design. Though if you have a decent monitoring setup or some good headphones you should be able to hear the difference. You might find it easier if you load and line up the racks in your DAW and then switch between the songs as they're playing.
5150 has the biggest low end
5150II has slightly more going on in the upper mids
5150III is overall a bit brighter than the other two
They could all be tweaked to sound exactly the same though I reckon. Which to me makes the 5150III not worth the extra money it costs imo.
Now if we could get some Bugera's into this comparison that would be verrrry interesting.