received my nimbus us4+ last week. Overall, the unit sounds good. however, my experience with the unit has not been entirely positive. The sound is largely neutral. I always felt my ears were sensitive to differences in equipment, but I have to call it like i hear it. In a blind test, there is no way I could tell the difference between this amp and the Headamp GS-X MK2. They sound virtually identical to me, regardless of headphone. I tried many headphones. If I had to pinpoint a difference I might believe the US4+ has slightly more deep bass and maybe the GS-X has very slightly more midrange ...but I really think the difference is so subtle that I'm not entirely sure the difference isn't due to slight variation in volume or my imagination. like I said, in general I hear absolutely no difference in tonal balance, clarity, soundstaging, imaging ,bass tightness, treble sparkle, timber, or any other aspect ...well, except for one pertaining to volume control. I'll get back to that in a moment. I guess it shouldn't be entirely surprising that the GS-X MK2 and the US4+ sound similar to me. They are both excellent, powerful, and in my opinion, try to present a neutral and transparent sound. Here's the thing. The GS-X MK2 is considerably cheaper. So I find my self debating ...is the US4+ worth keeping? For my purposes the answer may be yes. I wanted something that would perform in an alternate location with the performance of my main rig. I wanted something that would maximize the potential of my headphones, including the headphones that are hard to drive. I also like the one-box solution. This makes it easier to setup quickly. I also think the US4+ sounds good shortly after powering up. It may improve in the first 30 minutes or so, but by in large, I do not hear any short comings when starting cold. The US4+ also offers two features my other high-power high-end amps do not. I appreciate the balance control and the remote (which has not yet shipped). The balance control is helpful when the music is imbalanced or when a headphone may have a very slight imbalance. Incidentally, the balance control is somewhat subtle. It doesn't push sound all to one ear or the other, but it is effective enough to compensate for imbalances. The other feature I like is the remote. The remote would be useful if I am listening in my recliner and I do not want t reach too far every time I need to adjust the volume.
But here is my biggest issue - the volume control. The relays click at certain points. I do not think the volume control should be making noises on a 5K+ amp, but I can live with it as long as I know the unit is not broken and as long as it only happens at two spots when turning the volume control. With factory settings it happens at 9 o'clock and around 11:30 on the dial. It's not loud and it's brief. Except the other day I was listening and the click started to happen continuously without me adjusting the volume. To stop it, I simply moved the volume control a little. And then a day later I had a different experience. I heard the clicking more prominently (sounds like static) throughout the entire spectrum while turning the knob. I had been listening to the Susvara and had high gain on the prior day, But today I began listening to the Quad headphones with the same gain setting and (using a line-out on the Meridian Explorer 2 DAC). Anyway, the point is that the volume control is problematic. In addition, I had another issue which admittedly, I could not repeat but it is serious. I was listening with one headphone, pulled it, lowered the volume all the way, inserted a new headphone and the volume was playing as though I hadn't lowered it even though the knob was all the way down. As soon as I touched the knob, the volume reduced appropriately, This could have blown my ears or my headphones in the wrong circumstance. For now, I consider this an anomaly as I did not reproduce this, In fairness, I also did not turn the phone output off when swapping headphones ...not sure if that would have changed anything.
To conclude, I'll point out that I only have a few days with the Nimbus and it is an outstanding unit. It's certainly as good as any headphone amp I have heard. I question the value, but I do not doubt the sound quality. It delivers. It does not standout from some of my other high-end amps, even some considerably cheaper, but as I said, I am not sure it should. As long as the unit strives for neutrality, has sufficient power, is of good quality, and is a good electrical match, most units should sound similar, let's say +/-5 % - just my opinion. I realize others may disagree, The only amp I ever heard that sounded remarkably different (in a good way) from other amps was the Apex Teton ....but that's a different story for a different thread.