For what it's worth, malteaser's, from much of what I have heard about the M2U - and I haven't heard it myself - the $30 price difference is well worth it. The circuit of the M3 is basically exactly the same is in the new 100W Vidar amp, just scaled down to M3 size and after 20-25 hours or so on my new M3, I'm absolutely loving it! The HifiMAN HE400S cans are so smooth from top to bottom, not bright, not dull, the top end is the finest I've heard from any headphone amp in the last 26 years or so - and I've not heard all of them, just had a lot of experience, including building several of my own - midrange is even and is not disjointed at all moving through into the highs, which are extended and very detailed with superb focus. Frankly, I'm stunned with this amp! The only cans I don't like with it are my Sony MD-R1s, their frequency imbalances are shown up immediately, being far too bright and forward. Heck, this amp even drives my 600ohm Senn HD540 Reference cans which are 28 years old! Not perfectly but amazingly well, while not altering their own frequency quirks that I know so well. That's a great amp in my book. In saying that, I totally get "each to their own"