My setup sounds really good to me:
Aune M2S > Audioquest Victoria 3.5mm to RCA > Nuforce HA200 > Cardas Blue Litz cable > HD650 > Brainwavz angled velour pads (angle of the pads, imo, enhances soundstage, while the super thin fabric of these cups allows me to remove the stock foam insert of the HD650, minimizing the material between the driver and your ear).
For portable use I use M2S headphone out to Dunu DN-2000J. That sounds great, fantastic even, but the desktop setup definitely sounds better overall, although I haven't done extended listening with these particular setups. Although there is a slight hiss through the headphone jack, it's not dead silent It's insignificant, if anything's playing or there's any background noise in the room or it's not otherwise silent and you're listening for it, it's unnoticeable. What you hear me saying is that for portable use it's a nonissue. However, I don't recall any hiss at all through the line out, unless of course it's in the music, but otherwise it's dead silent afair.
My argument here is that you might want to consider getting a quality DAP to use as a source for your HD650, then you can simply get some nice IEMs or other portable headphones (actually you *could* use the HD650 portably, if you really want...) and you'll have your entire collection everywhere.
I find the HD650 responds well to the M2S, perhaps because it is an uncolored player that allows the HD650 to flex its muscles. My Fiio X3 (1st gen) does *not* do the HD650 justice. I mean it sounds good of course, especially via the line out, but the M2S really allows the details and imaging to shine. I didn't have my Fiio X5 (2nd gen) at the same time as the HD650, although from comparisons between the X3 and individually the X5 and M2S, the M2S seems simply more uncolored and generally more accurate than the X5. The X5 sounded great with exceptional detail retrieval and liveliness, clean across the entire range, but the M2S comes across as more articulate - faster and more precise in general. It also makes the X5 seem colored, in an attempt to overshadow its shortcomings. It would be interesting to do some direct A/Bing.
Unless you can get a good deal on the M2S like I did, you may want to look at the M2 or M2 Pro, I've read great things about those as well. In fact, a video reviewer on youtube noted they heard no difference between the Pro and the S, and since I've seen a few people running with that, although I've not heard the Pro. There are plenty of excellent portable DAPs these days, though, so I guess all I really may have done was add a lot more options to consider. <_<.
