Well,....I, unfortunately, have no advice to give......only understanding.
When I first fired off my new M50s 'bout a year ago, I was similarily underwellmed with the bass output.
.....guess I just 'spected more.
Being an M50 and M3 owner gives me an advantage; I can be expected to have a few idiosyncracies. So I can go off with impunity and do the bizzare; such as:
Using the bass control to add a few db down at 50hz, and a couple more in the receivers built-in EQ at 80hz to bring the bass up flat to my-sub-in-my-room, giving me just what I was looking(listening?)for. I had a great sounding integration, with the crossover set at 60hz!!
(yeah, the M50s and my M3s for center and surrounds like going down to 60, they tell me so)
But then.....something strange started to happen. The bass from the M50s slowly began "getting stronger"("ear" break-in?) and I began dialing out a bit of boost, little by little.
When all was said and done; I still have a few db dialed in to bring the M50s(which are about 3-5db down at 60)up to the sub level(which is now about 4db lower than it was at the start!!)
The bass from the M50s is better down to 60hz then my sub is at 60 on up!
The bass that I first perceived as weak, has now shown itself to be tight and clean and only needing just a little augmentation to match-up with the sub.(it took a year, almost, to get to this point, cause, to confuse things even further, I purchased a new larger(but still electro and mechanicaly happy in the 1.62cu ft box)sub driver to install in my ancient 1946 era movie theater plywood box) But yes; in the end I still have to add some stuff in the receiver to bring the mains up to where they sound good "to me"!!!
All is not yet complete however
While watching(listening)to "War of the Worlds" for the first time a few weeks back; I had the speakers, including the sub, singing away when at one point the Wife thought the sub was on fire!! She could smell burning wood!!
Turns out it wasen't burning, but we could indeed smell the internals of the sub box. The new driver was so overwelming the poor old plywood container that it was acting like a giant air pump evacuating a major percentage of the internal air volume.
Time for a new larger box, time to start all over again on the integration!!