A local Swiss firm mods it for output stage only. Not sold outside Switz though, unless perhaps an individual special export order is made.
Just got off the phone with them and they explained the mod to me.
They think the stock M51 is already great and has some small points of perfection (neutrality/low colouration, especially), but after the mod you no longer think of the M51 as digital, you think analog. The mod takes a great Dac to one class higher.
So, what IS the mod? What I was told was that its a mod to the output stage/analog section only, no PSU mod. I asked about the PSU and they think its fine and no need for extra shielding, etc. The output stage already has some high quality opamps, etc, but the change some components and and some other arrangements to get more dynamics, a warmer and smoother treble (more open yet more natural), an improved structure to the lower mid bass....better defintion, etc. After 5 seconds playing head to head, they expect you can hear the difference between stock and modded.
They wont give me more details than that. I hope this sheds some light on it.
