All ins and out comes in detailed review.

Great review sir <3 I can't wait for you to visit India and I get a chance to audition this beauty <3

For those who already own the m12, would you recommend the m15 as a replacement/upgrade over the m12?

Significant upgrade technically but M12 is still very capable for its price (I prefer M12 to ifi hip and idsd signature even). Unless you have demanding headphones or kilobuck grade IEMs, M12 is ok. M12 is slightly warmer than M15.

I'm looking for something that will sound more like my desktop Marantz HD-DAC1. When listening through my m12 with Sony Mdr-z7s, it sounds relatively flat and without depth and soundstage compared to the Marantz. Do you think the M15 might bridge this gap somewhat?