Could I use the E7 as a DAC and buy the m-stage as a amp or a Schiit Asgard? What is your opinion on this kind of setup?
Well, you can actually get a line-out adapter for the Fiio E7 to use it as a DAC. I have one. Using it with a nice amp isn't something I would bother doing though.
Think of it this way: When you buy audio gear, the quality of your sound is ultimately going to be limited by the weakest component in the chain. An extreme example would be if someone bought a great headphone like a HE-6 and combined it with a great amp like a HPA V200, then used a cheap mobile phone as the source. The headphones and amp aren't going to make the mobile phone sound better, quite the opposite, they will reveal all its flaws. So someone has effectively paid thousands to listen to how bad their mobile phone is.
Same goes with the E7, it's a nice portable DAC/amp for the price, but by upgrading everything else in the chain you are basically ensuring that the DAC in the E7 is the limiting factor. Now say you'd gone and bought something like the DT880, which is great value. Then, maybe if you already have an E7, getting an E9 or something else cheap as your amp would make sense, especially if that's the best you can afford. The DAC and amp would still be the limiting factors, but that's OK because it'd still be a nice budget rig. You could stick with that rig, or investing in a better DAC/amp down the track would be a logical upgrade.
But you already have a PRO900 and are talking about investing up to $1.2k in better headphones. My opinion is that investing in high end headphones and placing something like an E7 in the chain is really going to sell those headphones short. The DAC itself will not provide the extra detail you have paid for in the headphones. I myself have a Fiio E10 at the moment which I've been listening to my T70 through, when I go home and plug the T70 into my main rig, which is an STX and a Einar, I can hear things that were missing in the E10. And an STX isn't even all that amazing itself, it is a hell of a lot better than the DACs in the Fiios though. In fact, I'd sooner upgrade the STX than purchase more expensive headphones.