HM-601 Music Players

The Hifiman HM-601 is my favorite portable player for the following reasons:

*Sound quality--it has a warm, smooth, very musical, very analog sound. Detail and clarity are good, but this isn't the best player to listen to an audiobook, perhaps. Instead it's geared for music, and it's extremely good at it.

There is an effective 5 band eq if you want it.

*Soundstage--it's the only player I have with a soundstage that rivals my best desktop amps. It's very satisfying in this regard.
*Power--this player has enough power to drive full-size dynamic cans with as much power and authority as you could want. My Q701 is my favorite pairing, to take advantage of that soundstage. My 250 ohm DT880 also sounds excellent. The 601 does not power my full size planars very well (HE400, LCD2, Alpha Prime), but that problem is easily solved by using the line out with another amp. It does fine powering the Sine on its own.

There are no major downsides, but there are a couple less than perfect issues. The UI is completely non-intuitive, but it does have its own logic to it, so you will be able to learn to accommodate it without too much suffering. There IS a Rockbox port for the 601 (but not most 601 slim models). The install (and especially extraction) processes are more involved than most, so be aware of that.

It doesn't seem able to play a 24-bit wav file, though 16 bit is fine.

It doesn't pair well as a source for my Stax Lambda Pro electrostatics, in my current opinion. I thought it did at first, but it's lack of detail becomes apparent to me now. That is a unique situation, because it sounds great using it as a source through my desktop amp with just about every other pair of cans I have.

In all it's somewhat of a bare-bones player, with no digital in or out or other fancy features, but it plays beautiful, beautiful, music, and that's why I use it more than any other.

If you are after good sound this is all you need.  There is no need to carry a portable amp. There is enough power drive my 250 ohm DT-990's with power to spare. The sound is full and lush and takes my EIM's to another lever. 
I have the RE-262 and thought they were good. Matching them with the HM-601 brings out their full potential. Tighter bass, crispier highs, depth and wider sound stage.  Over a very crisp clean sound.
My co workers joked around when they saw it and didn't take it a serious DAP saying it looked like it was from the 80's., However the shocked look on their face after they heard it, said everything.

Very nice player, it is my second favorite DAP.
But will be my best player if I update it with RockBox, and that's if we have a stable version!!!