Nobody is a strong word for assertion. Opus, being one of the best, if not THE best lossy codec for audio[source]
certainly gets its share of attention. There are 7 bilion people on the planet, I don't think it's just me. No other codec can get you that much subjective, artifact-free quality on low bitrates. Some consider even 80kbps transparent. I personally prefer 140kbps.
How is WavPack terrible for portable audio running on battery??
It's way more CPU efficient than APE, which is supported natively:
APE (lowest complexity):
Decoded length: 2:55.907
Opening time: 0:00.000
Decoding time: 0:03.025
Speed (x realtime): 58.151
WavPack (lowest complexity):
Decoded length: 2:55.907
Opening time: 0:00.001
Decoding time: 0:01.121
Speed (x realtime): 156.725
Better is relative. I managed to test lots of headphones in my time and those that I settled with I personally consider the best ones. In a few weeks, I'm about to put HD800 into test against K240DF among others. I'm fairly certain AKG won't be far behind.
There are quite a few methods how to verify this "DAC sound" analogy. One of such being null-test, which unquestionably reveal any difference, if there is any to begin with.
Did I came out as unkind? That certainly wasn't my intention, nor does it appear in 2nd read.
Null-test, unlike method which cannot be discussed here, undeniably reveals any difference in sound between two or more devices, if there's any. The gist is subtracting one sound source from another, so you'll end up with just the difference. That's the safest, most accurate method of telling green apple from red one.
May I ask how did you perform this comparison? If the gap between changing sources is too wide, human auditory memory start to deteriorate just after 0.2 seconds.
Clip+ runs 15mW into 16Ohm load. That's not bad given its specs. Due to Clip+ size, one can't expect stunning crosstalk performance into low impedances, that's physical limitation which cannot be avoided other than making the DAP bigger. But even Clip's figure is good enough.
Last rockbox builds are capable of overdriving the amp into 1.1Vrms region, which is enough for casual listening in K240DF or K601.
OGG-Vorbis != OGG-Opus
WavPack != RIFF WAVE
Opus is quite different from Vorbis, and has much better efficiency, if not the best out of all lossy codecs.
WavPack, unlike FLAC, APE or Apple Lossless support both IEEE754 and in new iteration even DSD64.
As for X5-3 having Android, that I'd personally consider a disadvantage. Given how is recent Android limited (only MTP mode for USB access), how inefficient (at least 2GB of RAM needed vs. 2MB for Rockbox - that's one magnitude higher!) and unsuitable for audio overall (only 44.1k and 48k are supported natively).
My "new" setup running Clip+ into opamp-switched (TLE2062/OPA2277) Objective2 requires just 0.15W and last around 150 hours on single charge with target battery design. Without O2, Clip runs on 0.04W.
Compare that to X3II which needs 0.78W into line-out and 0.9W into headphone out.
X5II needs 1.27W
Those are some significant differences.
EQ in Rockbox are miles more exhaustive than the "preset" stuff in Fiio DAPs.
Centre frequency can be individually adjusted for each separate band (out of 15). All those bands can go up or down by 6dB in 0.1dB steps.
Bell width (Q-parameter) is also freely adjustable.
All this is not possible in Fiio's FW. There are just 8 bands with preset frequencies and width. Furthermore by activating the EQ, the volume automatically shifts down by 6dB even if you're just using the EQ for attenuation.
I would strongly encourage you or anyone else to revisit the $20 - $40 Clip+ with your mind free of judgement or McGurk effect. If it's that bad, it wouldn't be popular in the first place.