I'll give it a shot.  It's odd that a third party player would work while their partner's doesn't.

Documentation for new firmware mentions fixes for Android devices not detecting the DAC - however even with v1.20 my HTC M8 running Marshmallow is not detecting P90SD in Onkyo HF Player.

  DAC is very different as well, and while 5102A in HA-P50 is also a BurrBrown (with quite similar signature), it is an audible step down from newer 1795 in P90.

I tried mine through Wyrd and iFi iUSB, with either device DACportHD shows an improvement.  However, it is more pronounced with Wyrd, which seems to indicate that either my USB source quality is not up to par (which is likely, as I am using a rather dated laptop), or this DAC's USB chipset is...

I'm using a big Slappa case ​(SL-HP-07), seems to fit well.

Album and Artist names need to be perfect - the alternative is to create playlists for every album and navigate that way.

No, the UI has not changed at all, and I doubt that it will - the capabilities of built-in processor are quite limited.  After all, this is a DAC/amp first, and a player second.

​ Are you looking for microUSB OTG cable to connect it to a smartphone? then this could work for you, assuming your phone provides enough power over USB

1 - no, controls do not work when the device is connected as a "slave" to an Android. 2 - no, there is no L-R balance control on P90SD.   Between P90SD and X5mkII, technical differences are not very significant, DAC and output stages are quite comparable, and even the sound signature will...

Since both Slim and HD are Class A amplifiers, power consumption will be constant, regardless of the actual load of the headphones.  So expect 450mW + DAC (about 170-190mW) for Slim, and 775mW + DAC (almost 1W total) for the HD version.   Requirements for AK4490 DAC might be lower or a bit...

I have a DACport HD, and even when used with a laptop (Surface Pro) extra load on the battery is quite noticeable.

In general, Class A amps and portable use are not a good match. DACport gets very warm, regardless of the amount of power actually required by the headphones - so you will be using your phone's battery to heat the space around you.

The chip in DACport HD is Asahi Kasei AK4490.  The same DAC chip is utilized on the new Schiit Bitfrost, recently released TEAC UD-503 and Astell & Kern AK380 - among many others.   I was able to play back DSD 5.6 files with no conversion (except DoP encapsulation) from PC - but that appears...

 Yes, this device supports direct decoding of DSD stream up to DSD128.  The DAC chip itself does support even higher rate, and has a direct bypass capability for DSD stream - however, the current limit of DSD128 may be due to maximum bitrate of 384k / 32 bit in PCM mode, which is used to deliver...

I am quite certain that Snorry would gladly work with you on mods to LCD's. He has quite a following in Moscow, and the only drawback is his high expectations for quality of workmanship and precision machining of components and parts.