I've just read your review of the V6 with Z1, I'm a bit surprised by your opinion, I think the Z1 sounds pretty great with the V6, though I don't have the V5i on hand, but compared to the 49720HA that I've been using, it's a very noticeable improvement.
Well my result with the Z1 was, and it had already a Burson V5-i inside for a long time, that a V6 vivid didn't make a change. Not that I am aware off. I am 50 years old and maybe a little deaf already. I did not say the V5-i or V6 sound bad in a Z1, it does sound better than the stock opamp. Because I didn't hear any differences with the V5-i, I did not include the Z1 in my further testing. And the specs of the Z1 are a bit lower than my other Zishan players (but the sound is good). So I that was what I described. For me the V6 vivid in a DSD makes more sense.
49720HA, I think I have that one too, and maybe it is broken, I get distortion in the sound when I tried it.
My conclusion is, with Zishan players, a higher end opamp like Muses 01, 02, Burson V5-i, V6 vivid (compared to the vanilla stock opamp) makes a difference. But the differences between these higher end opamps in the Zishan players are difficult to expose. For me that is, with my old ears.
And I have moved further, something else has my attention. I have this Qudelix 5K DAC. It has a EQ in the software, and a list of presets for a lot of headphones. The EQ compensates the flaws in these headphones, the sound is corrected so it is more like the Harman target curve. With the EQ engaged, my Iphone with Tidal, and the Qudelix, gives a more pleasant sound than my Zishan players. My AKG 271 mk2, Beyer Dynamics 250DT, Grado SR80, Dali IO-6 all sound better with this EQ. So it it hard to go back to the Zishan players for me.