Well, after reading all the discussions, I've decided to dive in and order some of the cards mentioned here!
I had my doubts about SD card making any difference. My work involves mixed signal circuits (digital/analog/RF) and I have dealt with error correction codes including ones used for deep space communications. I highly doubt that bit errors are contributing factor to the sound quality or else our file transfer using SD card will be incredibly unreliable. We don't hear people talking about corrupt music/photo/video unless the card is broken.
I was reading the Sony engineer's interview who developed their audiophile SD card. He mentioned one very important factor - noise injected from the SD card. He claimed that certain SD cards inject higher levels of switching noise resulting from memory reading and other "control" circuitry included on the card. Depending on the DAP's architecture, those injected noise can make its way to the audio path, get amplified, resulting in different sound quality. For Sony's card, they apparently found a quiet memory element, or ways to reduce noise injected to the SD reader, or both. I believe this to be the most plausible explanation.
There are also people who claim degaussing SD card is just as effective. Perhaps it's a worthwhile experiment as well
Now the big question with 3Max is: Can the noise from SD slot make it's way into the audio section? The power supplies are isolated and I assume the digital and audio sections are isolated. Then how much noise can make its way to the ear? Perhaps a galvanically isolated SD slot could be the future. I dunno.
In any case, I'm going to give this a try and report back