The 990 is very bass heavy. The MA900 is well balanced. I'd say it's more fair to compare the bass to the 880. What you lose in bass, you gain in much better mids, and way less treble fatigue.
The MA900 is still a very fun headphone. It also responds well to bass boosting. It sounded like magic with the E12's bass boost.
I MADE MYSELF TURN THE BASS BOOST OFF. I thought the MA900 with the E12's bass boost was better to me than X1's bass at default.
I frown at bass boosting because it spoils what i expect out of a headphone when using them from other amps that don't have bass boosts. So instead, I let myself adjust to a headphone's own sound signature without a bass boost. I'm quite happy with the MA900's bass. There are times I feel the X1 has just a bit too much bass for some music. The MA900 has the right amount of bass for basically all purposes. Never too little, never too much.
Of course, I wish it had better sub bass (also wish the same out of the 990, X1, and others), but sub bass isn't a thing open dynamics can truly achieve.
Well. The more you talk about this, the more you tickle my fancy. Guess I'll have to try it with my E12's sub-bass boost, and compare to my Q701
Honestly, The Q701 is a gorgeous headphone. I love it and agree from experience that it scales well with upgrades to the rest of your setup, and I am quite satisfied in all areas between them, my M-100, and my IEMs. Now, but, however, graaaaugh! the highlighting points of this Sony tick pretty much all my boxes: yep, I worship at the altar of soundstage, lust for supreme marathon comfort, have feet that are motivated by midrange, pass rock & roll adolescence more every year so I want less and less fatiguing treble and bass (tho TBH always love near-neutral sound, my Ety's were my first hi-Fi headphones and remain closest to "monitor" flat rather than consumer-shaped), etc etc... And, I haven't said it outright before now, but I'm sure you've guessed: I have a certain "amour" for the value sweet spot, spending the least possible money for the most gain. The MA-900 doesn't cost much less than the price I paid for the Q701, but add to that difference how picky the Q701 can be (and trans-mutational it can be) about amping, especially to get closer to the apparent potential soundstage.
I picked up the Q701 because they have a reputation as the greatest soundstage (well, after the AD700 and K701, and HD800/Stax too I guess, but the Q has from all accounts more even frequency balance) and... well, they have a unique look, at the same time serious and retro-modern. A part of me wishes the Sony aesthetically looked a little more exotic than enlarged $30 headphones, visually say it broke the formula to be better. The same part of me that saw Art as a viable college alternative after Mechanical Engineering, and I ended up compromising and studying graphic design.
Anyway, left-brain/right-brain will continue to argue, but I can say I'm already in greater anticipation than I was about the HE-400 (for my tastes). I'll get the test playlist ready, and mentally prepare for a careful review. Again, flattered to be included.