I've joined team "no foam padding" with the FA-011.. it definitely opens up the mids, but not night and day in any sense.. a good mod for sure that I'd highly recommend, though.
Running the FA-011 through the paces with the Audinst HUD-MX1.. what a fantastic pairing! As I stated earlier, the 011 was surprisingly decent unamped from my iPhone 4, strictly speaking on sound quality (well controlled and clear for running out of a DAP at full volume but it wasn't particularly loud or impactful). With the Arrow it was excellent.. the MX1 takes it up another notch over the Arrow.
The bass quality improvement is apparent right off the bat: tighter, quicker, and better defined.. without losing any of the irresistibly smooth punch. Mids get pulled up in line with the treble. Amp the FA-011 adequately and vocals sound heavenly. Smooth but detailed, natural timbre, not forward or recessed, warm or thin.. simply in line with other the instruments in the midrange. Treble sounds deliciously crisp and extended without a hint of sibilance. Even in very busy, treble-heavy sequences, I heard no smearing or grain as cymbals, high pitched female vocals/samples, and other sounds were heard clearly and reproduced with an effortless quality as they coalesced. The treble is another area I feel the MX1's synergy with the 011 becomes apparent.. I listened to some well mastered music with ridiculously crisp, sharp treble.. and I found it fatiguing after some time not because of the 011's treble shortcomings (or lack thereof), but simply cause it reproduced the treble with an accuracy, sharpness, and crispness intended in the recordings that hit the limits of my hearing. With the MX1, the U shaped sig (more prevalent when unamped) practically disappears.. it becomes a much more balanced, even smooth curve: resembling a shape between a U- & V- curve that's heavily flattened and stretched very widely. This is evidenced by how much clearer and more forward the mids sound, the tightening of the bass, and cleaned up treble.
The FA-011 has a voracious appetite when it comes to amps.. it simply LOVES power. Typically, I never turn the MX1 past 10 o'clock with my phones.. venturing to 12 o'clock when I'm in the mood to get completely immersed in music or rock the f*ck out, but even that gets fatiguing and painful.. with the FA-011, I can approach the tick between 2 & 3 o'clock when the SPL levels become painful.. but even there, the FA-011 simply HUMS along.. bass remains tight, midrange chugs along with astonishing clarity and balance, the treble shimmers.. and the sound is simply hypnotic and immersive.
I'll probably hit up a craft store to get replacement padding which I'll mod to leave the driver grill uncovered but pad the area where my ears make contact with the plastic.. I don't have any discomfort with my ears touching the plastic after the foam padding was removed.. but it certainly felt cozy having some padding there.