Got to spend several weeks with the M4U1. To cut to the chase, I didn't choose to keep them. However, I must admit that it wasn't easy to let them go, as they stood up very well to some other, more expensive headphones that I was comparing them against.
To begin with, they were quite comfortable for my head. My initial impression was that they were a solid, but unremarkable sounding headphone. But the more I listened to them, the more I came to the conclusion that I was mistaking the lack of exaggeration in some frequency range as a lack of energy or fun. But as I compared to other highly regarded competitors, I discovered that the M4U1 was a very solid sounding headphone.
The M4U1 didn't have the bass quantity that the X2 had. They didn't have the energetic mids that the SR80e had. And they didn't have the bright detail of 400S or 400i.
What they did have was smooth but clear highs, without simblace. They had clean, clear, and detailed mids; And the bass also had generally decent clarity and articulation - albeit lacking a bit of impact and maybe somewhat flat or recessed.
Overall, I thought that they were excellent headphones that I could have been very happy with. In the end, I ended up keeping the 400i, which I thought were better suited for the type of music that I listen to (most genres of rock), rather than due any definable flaws that I found. I thought that the M4U1 was very neutral, but just didn't have the mid/high energy, or the bass impact that the 400i had. Still, I thought it was worth mentioning that I had a nice audition with the M4U1. I think that they would do well for many.
Of note, the M4U1 was more sensitive that the 400i. If I didn't have a proper headphone amp, I may have kept the M4U1 over the 400i.