I don't want to sound like a broken record, but as far as PORTABLE solutions go, I can't think of a setup that I've been told about, read about, or listened to that drives the HD-650s better than the iDSD Micro without including the Chord HUGO ($2500 USD). For all other headphones that I've listened to, the amp in the DX90 was amply-powerful enough on its own.
In my book:
DX90 alone = Acceptable for the HD-650s, but a little anemic and not quite as wide a soundstage or as clear a separation as the next options. Still TONS better than my iPhones or iPads by themselves in terms of volume, SQ, Dynamic Range, and responsiveness for both PCM (MP3s) and DSD files. Though the DSD files make more of a difference in terms of improvement over the Apple devices natively.
DX90 with FIIO e17 = A little more power and ability to pull more BASS, (or Treble, but I never hear a need for it) if desired, but the DAC used is still the DX90s. (Which is pretty darn good.)
DX90 with iDSD Micro = Truly impressive! The best portable solution I've heard. Soundstage is "appropriate" for classical and pop music alike. (By appropriate, I mean the instruments surround you without feeling artificially stretched/expanded. Just "natural and wide".). There is also an added level of clarity to the HD-650s that I don't get from the previous portable solutions. I guess it's what many call "the Sennheiser veil". The veil is not noticeable to me at all with this combo unless I compare it to a powerful, dedicated, headphone amp like my SCHIIT Audio ASGARD 2. Even the headphone out of our Panasonic surround sound rack unit in our home theater doesn't make them sound as clear as the iDSD Micro or ASGARD 2 does.
IPhone 4S, 5, or iPad Air with iDSD Micro = The same as the DX90 with the iDSD Micro, but there is MUCH less expandability to fit all the songs I want. The bonus of being able to drop 128GB microSD cards into the DX90 is HUGELY superior to me over the 32 or 64GB SHARED storage I have on any of the Apple devices we have.
In hotel rooms/visiting family and friends or mobile moving around home: use HD-650s
Simple setup = iPhone 5 with iDSD Micro - lightest combo, but no more than 2 or 3 Albums fit along with the photos and video, and other apps taking up space....
Preferred setup = DX90 with iDSD Micro - slightly bulkier footprint and just slightly heavier, but enough storage space between the 8GB internal memory and the 128GB cards to carry every album or song I care to.
During Travel: use other headphones
Simple setup & Preferred setup = DX90 alone. I never use my HD-650s in public as they are open back, and not ideal for public listening. Bass FREQs for in-ear sound isolation, or Photive PH-BTX6 headphones are driven beautifully by the DX90. I never need to turn the volume past 11 or 12-o'clock. The Koss PortaPros leak more sound than I would like, but are great in the airport if you can find some space away from others. I would feel rude using them on the plane though.
