Just finished listening to the Mason FS and Indigo and here are some impressions using my Lotoo Ti and the Attila cable since I sold my M3 cable:
1) the Mason FS has a smoother presentation in the mids and trebles whereas the Indigo has an elevated lower treble that more readily reveals recording quality deficiencies. It’s sharper sounding while the FS is creamier.
2) the Mason FS has a headphone like stage but the Indigo is able to tease out more micro-detail behind one’s head giving the sense of a deeper stage. Could just be the difference in the shells and how they make contact with one’s ears.
3) the mid-bass on the Indigo is punchier than the Mason FS.
For half the price of the Mason FS, the Indigo is a way better value, but the FS and its tonal signature is uniquely appealing.