Take a look at this graph
of the Era 4. The - 3dB roll off appears to be around 80Hz (I'm eyeballing it), but there is a pronounced hump in the 100Hz to 200Hz range. That could account for some of the perceived difference.
The M3s actually go lower (closer to 60Hz), and have also have a hump, but again lower than the Era 4 as seen in this M3 graph
[Crud, that only gets you to the M3 page, but click on the link labeled Graph in the specs section]. On paper, you would think the M3 should outperform the Era on the low end. But there may be some midbass emphasis in the Era 4 that you prefer.
Do you really have the M3s set up in the (exact) same place you had the Era 4s? As was mentioned, you could definitely move them closer to the wall to gain some reinforcement. But it could be that you just prefer the sound and character of the Eras.