This morning I took delivery of my US4+, which I was able to get with a nice discount along with a trade-in cash-back for my V280. Before sending my V280 back to the shop, I was able to do some comparison this morning. I tested with an LCD-3, HD800S, Utopia and a brand-new LCD-5 (also delivered this morning), all from my Lake People RS 06. I'm not great at this at all, so do take everything with the well known grain of salt.
In general, I would say the V280 and US4+ are more similar than they are different. Tuning to me is very similar, and while I was a little worried that the Niimbus would brighten the sound of my headphones, I find that it barely does, if at all. I do feel like the US4+ give up just a touch of bass quantity.
That said, when going back and forth between the two it's quite clear to me that the US4+ is the more resolving of the two. It has the better soundstage and placement within the soundstage, and teases out more details compared to the V280. Each time when going from the V280 to the US4+ I noticed the sound just cleared up and was cleaner and more detailed overall. It's not like the V280 has become muddy, but the US4+ does really up the level.
It wasn't until I plugged in the HD800S that I felt like the V280 was offering just a touch more warmth and musicality to a headphone that really needs it. I didn't really have time to do more comparisons between the amps with the HD800S though to really nail this down, so might just have been my imagination. On the flipside, the HD800S certainly lose definition, clarity and detail when going back to the V280.
I did not find the difference between the two to be massive at all, but once you start driving headphones in the Utopia and LCD-5 category, I think it's a worthy upgrade. Though not necessarily a must, as the V280 is certainly capable of driving all of these headphones to very pleasing levels. Please keep in mind that all of this was done with a Lake People RS 06, which is nice little DAC, but higher end devices might emphasize the differences between the amps more clearly.
The only area where the differences are anything but subtle is in the look and feel of the units. The US4+ is clearly the more handsome of the two, with a more mature and high-end look. I never did mind the look of the V280 too much, but I'm glad for the updated front panel. Much better. I really like the LED in the volume knob, so you can see your listening volume in the dark!