I agree, the micro iDSL BL is great and I use it when I need a tranportable SE amp (it doubles as my SE preamp as well).
The i4s need a good amp (like the micro iDSL BL and even a nice desktop amp) and some say it needs some EQ; I do use ROON DSP. The i4s punch way above it's diminuative size and competes with over-the-ear full size headphones; some say it's a mini Audeze LCD-4. I've never heard the LCD-4 but my i4s knock on the door of my TOTL headphones via my headphone amp.
Back to the micro iDSL BLs. I would definitely like to see a balanced version, eg., 4.4mm balanced? I would forgo the ability to charge my phone (I have a separate battery charger for that) and it would also reduce the size.
Cheers!