Haven't really compared them on the XI1 yet. Normally, fast filters have more extended treble. A potential side-effect depending on synergy is causing less natural sound (disjointed notes, metallic/plasticky timbre) due to likelihood of ringing, so likely best used with dark IEMs. Slow filters roll off sooner in the treble could help with situations where you have a metallic or plasticky timbre or disjointed notes (by reducing ringing). I always start with Nonoversampling whenever the option is available, just to start on a blank slate.
Side note regarding the leather case (reason for test is others reported sonic differences from limited edition DAPs that used different housing metals):
Anyone can feel free to blind test the XI1 with and without the leather case. At least I am not spreading snake oil because most people got the case for free (plus I am not recommending the case anyway). Unlike $3,000 cables, everyone can blind test with the cheap leather case that they got for free.
To test, it is very simple (use stock OTG cable to test):
1) Find a partner to assist you with the blind test.
2) With the case already on, connect the ONIX Alpha XI1 to your computer's USB port and leave it suspended (hanging down) in the air. Then connect headphone.
UPDATE: Use High Gain (4.4mm balanced output) instead of Low Gain for the test (will make difference easily discernable). Use Low Gain for regular listening.
3) On foobar, play a song you are familiar with (I suggest a soft vocal track with constant bass and cymbals. This lets you easily identify if the bass starts distracting from the soft atmosphere).
4) Ask your assistant to play your selected passage at the selected timestamp (remember your observations).
5) Ask your assistant to slide the case off and replay the same passage (remember your observations).
6) Your assistant will now randomize it and you can begin guessing. State your guess to your assistant who will note down "correct" or "wrong".
7) Note your observations and repeat. Try to get 9 or 10 out of 10 to ensure confidence level is high.
8) Based on the final tally, you have your answer as to whether you perceived any difference in sound with/without the case on.
9) If the result is that you perceived a difference, then you can then proceed to test with the dongle flat on your desk, inside your pocket, etc. If the result is you perceived no difference, then there is no further need to proceed.
10) Blind test done.
My observations using the stock OTG cable + ZMF Verite Closed on Windows 10:
-Case on: reduced diffusion/denser notes, narrower/deeper soundstage (more holographic), grounded/less elevated cymbal trails, subdued presence region, more contained reverberations, punchier/deeper bass
-Case off: more diffuse, wider soundstage (left/right), more airy/elevated cymbal trails, cymbal trails linger and spread more over soundstage, softer/lighter bass
Anyway, this is faster to do on the fly than changing filters since there's no need to disconnect anything due to the case being automatically secured by the OTG cable between the dongle and the source player.
Listening blind tests are reliable as long as it's a personal result with high confidence level across multiple guesses, although the result is not universal since someone else can have different hearing.
