I've already got about six hours on the little stack so far and am actually extremely impressed.
Initially I had a problem with the Modi, it was picking up a lot of interference but I solved that by moving my router - totally silent now.
The Vali initially had awful channel imbalance, I had to push the MusicBee balance slider 40% to the right to centre things up. Strange thing is after a couple of hours that had dropped to 20% and it's now at 10% and still dropping, so I'm hoping that it will disappear after a little more burn in.
As for microphonics, I have a very small amount but it only happens when I first put the headphone jack in or really thump the case and disappears after 5 to 10 seconds anyway.
Absolutely loving the sound, compared with the Aune and the STX the bass is a lot punchier and cleaner and while the midrange isn't quite as smooth and creamy as with the Aune (with Mullard 6DJ8), it is a lot more detailed. I think it's the highs that have made the most difference, they are more detailed, crisper and cleaner than the Aune and much smoother than the STX. Overall the M & V stack seems to be a really good match for the HD 650's and because of that I haven't stopped grinning.
As for what headphones I'll use, it will just be the HD 650, AKG K702 and DT 990 pro, all of which are high impedance so no worries about the noise floor.
I think I'm going to be very happy with this setup for quite some time.