Part 2 of my review. Using Explorer 2 on a phone.
I am using a Galaxy Note 4 (5.0.1) with Denon D600 (25 ohm + short cable) and Klipsch X10. This
Y adapter from Amazon allows me to connect an external USB battery to power the E2. My phone can also power the E2 fine by itself.
Volume
Both my headphones and earphones are very easy to drive. My max listening volume using the phone alone would be around 9 to 11 out of 15. With the E2 attached, however, I find myself using the max 15/15 volume and still wanting a bit more. The volume is perfect at a quiet place, but it's simply not enough while on a moving train or car.
Tidal / Spotify / Music Player
3 lights on. Noticeable improvement using the E2 vs stock audio output. Listening to Tidal HiFi on my desktop sounds better with 44.1khz and 16 bit setting (1 light) than on my phone. All the settings are the same.
USB Audio Player Pro (free trial)
1 light on. Best quality using lossless files. The Audio Player Pro is like the Foobar for Android -- it allows the E2 to do all the processing.
With vs without an external battery
Biggest surprise of the day. I was mostly testing without the external battery because I can't imagine fitting in my phone + E2 + external battery in any of my pockets. After my testing, I set up my gear back to my desktop. I outright preferred the sound on it than on my phone despite using Foobar on desktop and APP on phone. Only the E2 should be processing the music in either set-up and shouldn't sound different. I went back to my phone and plugged it with an external battery, but from the 1A port instead of the 0.5A. The sound difference is not as obvious say from mp3 to a lossless file, but I feel that it's there notably on the bass.
Oppo HA-2 vs Meridian Explorer 2
The HA-2 is an amazing product with solid build quality, powerful amp, built-in batteries, bass boost, volume control, and low/high impedance option. The E2 is a smaller and lighter, but lacks those features. The E2 is portable in a sense that it's easy to carry. However, when it comes to pairing it with the phone, I would vote the HA-2 over the E2. The lack of volume plus the need of an external battery to make the most out of it is the E2's biggest downside. The HA-2 has no problem driving high impedance headphones including the HD650. Of course the E2 was never meant to be a phone amp/dac. Luckily I knew well that I will be using it with a computer, so it's not that great of a deal breaker for me. However, people looking to pair something with their phone should get the HA-2 instead.
UPDATE 4/20:
Included review using Tidal.
Volume level against loud background noise (train and car).
Short burst of static sound.
I compared the different set-ups by listening to parts of different music. Now that I've used the E2 for the whole duration of the tracks, this very short static-y sound occurs once in a while (~.25 sec bzzt).
I turned on airplane mode, switched wi-fi off, tried different music players and volume levels, plugged the external battery in/out... It still happens randomly. I'll try a different phone and see how it goes.
UPDATE 4/28:
I used the E2 on a different Android phone and no problem.