I think a lot of the comments in this thread are on target. I don't think the comments about soundstage or soundstage width are too useful since unfortunately the comp I put up had the L and R channels swapped on the littlebox.
But the LB does have a bit more of a low end to it than the V3 as people note. I had been thinking that was an effect of the LB, but your comments on the low end of the V3 do have me wondering about whether it is in fact the V3 with a slightly rolled off low end, and not the LB having a somewhat accentuated low end. Still, getting the low end where you want it is easy enough with post processing EQ, and I'm usually doing that anyway since live PA shows typically have too much subwoofer and low end action.
Beyond that obvious difference between them, I think jlykos' comments a couple pages back are right on. Though the differences that he describes to me are not blaring, but subtle. The LB presentation is not as coherent across all frequencies, but I find this a difference only real golden-ears listening will pick up. The LB is a bit more forward in the mids, which is actually a sonic signature that I prefer. And the mids, perhaps by being a bit more forward, can lean towards being a bit more strident, esp when recording PA's (in particular line arrays) that I find can be strident themselves.
I got my LB fitted with output transformers and as Jon suggested, these help the sound, and tame the tendency towards stridency. They also can get to some low end transformer saturation that I really like. I got mine with switchable output transformers, but find that I always want them engaged.
Finally, it is worth noting again that my LB is special as it is one of the few made with Burr-Brown instrumentation amps, as I had requested, so it is not the same as stock littleboxes being made today. I might try to undertake another comp using my LB with the output transformers sometime when I've got an indoor show to go to (outdoors I'd prefer using the extra channels for split omnis). But it would be good if someone else with a standard LB could do a LB-V3 comp sometime.