The M100 would not be overkill for you. You'd need an ADA-1000. Any more amp with the M100 would be too much for your listening distance.

The M5s put shame to your M60ti. I replaced my M80v2 with M5s and the difference in soundstage, imaging, accuracy and emotional satisfaction was immense. The M100s are even cleaner and more dynamic.

If you do get M5s, the ADA-1000 won't be enough for you.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated