I had the A12ts for over a year and absolutely loved them. I lost them and was going to buy them again until I heard the M4s. The A12t probably has the best BA bass I've heard. It almost feels like a dynamic presentation. In simple terms, I would describe them as being fun and analytical in a way, because you get lots of details w/ those Tia drivers. I'd say they probably are a little more detailed or at least slightly brighter than the M4s (although I don't feel I'm missing anything w/ M4s). On the custom version, from memory I feel that the bass has slightly more qty than M4. Overall, they are fairly neutral I'd say. The M4, is very well balanced overall, I really like what Fir Audio has done with the tuning of their dynamic drivers. They have excellent extension (maybe better than A12t from memory), but are very well controlled for DD. I heard the Empire Ears lineup and while I generally liked them, I had issues with how slow and bloated the W9 DD can get. The M4s have excellent speed and are so well controlled. Where I'd say the M4s and really everything I've heard from Fir make a huge difference for me was on the emotional and engaging perception I get from listening to music. I believe it has to do with them having that tubeless design, so I imagine there is more air movement and also the sound stage is more 3D and natural sounding in a way. I'd audition them if you could, but honestly, I'm not looking back at the A12ts which are great, but I believe Fir audio is on to something here.