Actually the HE-400 does not work well with the Magni. I have the HE-400, and tried it with the Magni. After I tried the Magni, Headfonia.com reviewed it and their reviewer Julius had exactly the same impression:
" I would not call the Magni a great match up for this Hifiman headphone, it just lacks the bass quantity to get the job done by itself." and
" Sadly, the HE-400 really showcases too much sibilance with the Magni for my ears. "
This shows you that the numeric power rating by itself is not sufficient.
Both the HE-400 work significantly better with more power and quality, but interestingly, this is even more true of the HE-400. You never hear this, because the HE-500 are so famous, that most people with the money for a good amplifier, buys the HE-500 instead of the HE-400.
I found that as well, the balanced HE 400 out of the Audio GD 10ES2 [same as my Dt 880] gained a little more sound stage and more define and focused sound over all, and both got a tighter and weighter bass.
More than likey yea, the differance is only marginal... we talking out of 20 songs you'll notice one sounds SERIOUSLY better
still... I like the MD for comfort and ioslation... utility wise it was great... I guess I just want to give it the BEST possible chance to sound well to my ears good.
THe biggest piece of advice in this hobby would be to enjoy your gear. IF you like it, then just simply enjoy it! If your MD out of the PA2v2 sounds good to you, then just enjoy it! Eventually you might have enough cash on hand to make it sound better, but again. If you like it not, don't worry to much about upgrades