I am listening to the 0404 connected to an iMac right now and it has worked flawlessly thru USB and toslink. I too have the 990s and they sing in this set-up. I even had the Headfive in this set-up at one point too. Liked it enough at the time, but have since liked almost every other amp to improve the sound more than the Headfive, even lower-priced portables! I know it is a little heretical, but the Total Airhead, for example, sounds fantastic in this set-up. It has wonderful dynamics and good bass punch and a huge amount of energy. Also, the iBasso P1 is very smooth, albeit a little more analytical than the Airhead in this configuration. The Yarland tube amp sounds warm and and round lovely too, but that would be more expected....
One interesting thing about the Emu is the way the "Main Output" can boost the level out to an amp. I have noticed no deterioration in quality even when pushing the output to max (not that I run it this way). The Emu seems to put out a lot juice for amps to work with. I have asked around about whether this means one can get away with a less powerful amp to drive higher impedance cans but haven't gotten a firm answer. However, when discussing the issue with Norm from GoVibe pertaining to my incoming purchase of a V6M he adjusted down the suggested gain when I sent him the specs on the EMU.
My impressions of the EMU hp jack are the same - good but a bit grainy. Powers the 990s well, but struggles with K701s. As for whether to upgrade the Marshmallows, I would think that after getting used to the 990s you are going to have trouble enjoying iems anywhere near Marshmallow price. I upgraded iems several times before, like about a trillion others, found happiness in the form of the ER4s.
Finally, glad to hear you are loving your set-up. ENJOY!
