You should be careful not to generalise!
1> See my post viewtopic.php?f=84&t=64350
2> The DR nano does NOT have an external power capability and runs just fine of the OTG connector
I could be wrong but from my testing I got a feel from the sounds produced that they were post dsp processing as they were horrible!!
Sounded fine on my cans!
4> I am not having much luck finding an Audio app that reliably supports the DR Nano DAC, however I have been trying a SUPERB sound app, "Neutron MP" This may not have a UI to everyones taste. BUT IT SUPPPORTS 24/96 bit decoding using its own decode engine. It sound great on some of my highdef flac files played through good quality headphones. NOT through my DAC, but using Neutron app only.
Here is some detail about Neutron:-
32-bit audio decoding/processing for high quality HD audio.
Audio formats: MP1, MP2, MP3, OGG (Vorbis), WMA, AC3, AAC, M4A, M4B, M4R, MP4, 3GP, 3G2, MOV, FLAC, ALAC (Apple Lossless), APE (Monkey's Audio), WV (WavPack), TTA (True Audio), MPC (MusePack), WAV, AU, AIFF, MPG/MPEG (audio only), AVI (audio only), iTunes/Windows Media inclusive except DRM-protected.
Modular audio formats: MOD, IM, XM, S3M.
Voice audio format: SPEEX.
Surround sound (Ambiophonic R.A.C.E. technology).
Crossfeed DSP (for better stereo music listening experience with headphones).
Rumble Filter DSP (to protect speakers from overloading with frequencies below 20 Hz).
Normalization (post EQ re-mastering of tracks).
Streaming audio: M3U, PLS, ASX, RAM.
HQ 4-band parametric equalizer.
Accurate gapless playback.
Resampling with 3 modes: fast, quality, audiophile.
Dithering (to improve audio quality by eliminating quantization in signal).
Crossfading (including manual track switching in playlist).
CUE Sheet support (including built into metadata).
Master and Preamp digital volume adjustment.
Media database grouping by: album, artist, genre.
Real-time multi-band spectrum analyzer.
Real-time RMS bar.
Android widgets and Lock Screen mode.
Movable to external SD Card.
Night UI mode with colorful audio visualization.
Album art (supported: PNG, JPG, BMP, incl. metadata from tags).
Album art visual effect based on currently playing music.
Sleep timer: 15,30,45,60,90 minutes.
Automatic key-lock with colorful audio visualization.
Highly customizable through Settings.
Smart CPU/Battery consumption.
Android v.2.1 (Eclair) and higher.
Only for ARMv7 compatible CPUs.
Optimized version for NEON compatible CPUs.
Display resolution 480x800 and higher.
Try it you may be pleasantly surprised!