Well I have to eat my words.... my HD 660 s impression was totally wrong. I totally botched the volume matching making the schiit stack considerably louder than the Play. I quite idiotically matched the volume of the peaks rather than matching them with a tone generator.I usually do this to save my hearing from getting damaged by excessively loud peaks but it was a mistake in this case. I had the Play too quiet in general taking the volume from 8 to 12 made a world of difference.
The positive of this is that it led me to the discovery that the Vivid Play actually measurably increases the dynamic range of songs. Once I got them matched at 500hz, I recorded the highest peaks of various songs and found that the play peaks 1-2 db higher than the schiit multibit stack. I had one outlier where it was 3db(!) higher. It also goes quieter I think but that is harder for me to measure consistently, looking at the sound level meter though the play is all over the place in volume, whereas the schiit stack changes less and less dramatically when it does. It sounds louder, even though they are the same volume due to the dynamics which is why I didnt notice the difference by ear.
Doing a properly matched AB test revealed the hd 660 s/Vivid play make for an absolutely killer combo. The HD 660 S already has outstanding dynamics and the play just makes it even more so. The details on this combo are simply stunning, now that I have it at the correct volume, its better than the Hifiman for everything other than female vocals/soundstage, which I just really enjoy for some reason on the hifiman, probably that planar clarity or slight brightness to it. It strikes the perfect balance between detail and harshness, I did the purple Lamborghini test with it and it would come right to the edge of where I would rip my headphones off and scream obscenities at Skrillex and stop right before the cursing began. Perfect. All the details there and none of the swearing.
The only advantage the Schiit stack had over the play is that the bass is slightly punchier, though the deep bass is pretty much the same. The tonality is also slightly darker and warmer on the Schiit Multibit stack, which may be a positive for some, it is not for me. The Play sounds neutral to me in its tonality though the frequency response is not I think, its a very well done effect its got going on. Instrument separation is simply stellar on the play and blows the schiit stack out of the water, a side effect of the increased dynamic range. The soundstage will also sound wider, again a effect of the DR.
I am really impressed by the 660 S/Play combos ability to make badly mastered albums enjoyable, I listened to the mastered for itunes version of Kiss Crazy Crazy Nights known for its plasticky/ overly clean sounding instruments and was really impressed at how much improved it sounded. THe original version of that album still sounds pretty poor even with this combo (bass guitars apparently werent worth hearing in the 80s) but the remaster did a lot to clean it up and the play finished the job. Bruce Kulicks guitar work really pops.
It even managed to make Iron Maidens No Prayer for the Dying sound surprisingly decent. It was literally recorded in Steve Harris' barn and....you can tell. The play makes it so you can dislike the album for the songs being bad rather than the mastering. Fear of the Dark sounds damn good, which is really surprising. Fear of the Dark was the first time they used digital recording and mastering rather than analog and...you can tell. It sounds like you are listening to a proper remaster off of this combo. The titular song gets almost as good as the live version is. If the bass guitar were mixed louder, it might even have been better than the live version, alas twas not to be.
I would absolutely recommend the Vivid Play to someone who is looking to make their lower DR music sound slightly more dynamic. I also bought a IFI earbuddy to try out the play with some of my lower impedance more sensitive gear and it seems to work great. I am surprised the impedance mismatch is not having much effect on them. I expected them to turn into overly warm boomy messes, but surprisingly they are not and the hiss is gone. Definitely a great investment for use with the play.