If you can manage to get the Magni to complement the Modi, you will most certainly be golden - it is an excellent value for what you can get out of these headphone for the price. The extra wattage really brings the HE--400 to life from my experience. I currently use the Magni connected to my sound card, a Titanium HD, but will plan on getting the Modi eventually. If you have a decent sound card, I'd say swap the DAC for the amp for the time being if funds are short. I listen to quite a bit of metal (Ulver's Bergtatt is melting into my ears right now), and I find my current combo works brilliantly, albeit with the occasional minor EQ tweak here and there.
As for an alternative, I've not had an opportunity to try many other full size headphones out yet, but I'd suggest looking into Beyer's DT880/DT990, Denon D2000 (if you can find one) or possibly Grado's SR225 or lower models (I have a friend who has these and is also an avid metal fan).
I love ulver! That is a good idea. I also listen to quite of bit of death metal such as defeated sanity. In the album psalms of the moribund I noticed that the headphones don't really emphasize on the punchy quality of the bass. It all kind of sound flat.