Thank you for the reply...I would be powering them through the 901, which is supposed to be a powerful unit in of itself.
That it is, I would recommend getting the Balanced Amp Card from Hifiman as that's the High Powered Amp card for the unit,
If that's not within your budget, I would see if you could purchase the two units from any vendor with a return warrenty, although honestly I think it will be perfectly fine
If your Jumping from something like a Closed Back Dynamic, the HE 560 and HM 901 are going to sound noticeably BETTER, I don't recommend you get the 400i since while it may "pair better" with the HM 901 it's still not the better headphone.
So I'd try to get the Amp Card for the HM 901, if you cannot do that then just enjoy the Headphone and DAP that you have, as yes the HM901 is a very powerful unit, that's it's selling point. While I think that using the HM901 Line out to a Balanced Amp would be a better sounding option, others feel the cost for said option defeats the pourpose of owning an HM901, and others feel the cost is justified
SO long story short, get your HM901, get an HE 560 and enjoy it, when you can afford to Try getting another amp and see how it benifits the set up sound wise! but the HM901 is going to be leagues above an iPod.