1x microSD + 1x SD would be an ideal solution for sure.
but Calyx-M's battery life is piss poor for its brick size so maybe they could've improven that aspect by ditching 1x mSD slot. i don't know.
SD card and micro SD card consume nearly no battery at all.
They have nothing to do with battery life. If you put 10 SD card slots and stream movies from all of them, then maybe but having 4 micro SD slots for music streaming will not kill battery since 3 of them will always just idle.
Also, micro SD card slot and SD card slot are not expensive at all.. they may cost $1 per each. This SD card adapter's price tells me that.
https://www.amazon.com/Transcend-microSDHC-Reader-TS-RDF5K-Black/dp/B009D79VH4/ref=sr_1_4?ie=UTF8&qid=1468953210&sr=8-4&keywords=sd+adapter
They have nothing to do with firmware complication either.......
See this $10 SD card reader. It can read 7 flash storages at a time.
https://www.amazon.com/Geekdigg-Speed-Reader-Slots-Combo/dp/B018S7ZNDU/ref=sr_1_11?ie=UTF8&qid=1468946138&sr=8-11&keywords=sd+reader
They charge only $10 for compatibility with 7 flash storages including SD card, USB flash storage, micro SD card, etc.
If they can make it for $10 on Amazon, it must cost only $5 to do at a factory.
That's why I'm questioning iBasso guys. If SD card slots and micro SD card slots go inside that bottom metal lid, there's no more reason to not implement more of them when DXD album is 10GB, DSD 128 album is 6GB. More storage is just better and it completes idea of a perfect DAP which can store infinite amount of high-res music.