Just got my HD6XX’s and they are excellent so far, almost... played them through my iPhone XS, IPad 4 and Marantz SR5014. I know a dedicated headphone amp will improve the sound but I don’t want to throw money away if I’d doesn’t accomplish what I want. Basically they sound amazing until there’s significant bass like an explosion, obviously open back headphones aren’t what a bass head has in mind but these just fall apart completely. Watched Greyhound on the iPad and played some Call of Duty modern warfare, everything is amazing until some demanding bass is thrown at the HD6XX’s. Question is, will a Schiit Heresy or JDS Atom solve this?
There is a not-terribly-difficult way to increase the bass from your new headphones. I've never done it, because I'm happy with the response curve of the pair of HD-580 that I've owned since the very early 1990s. It is interesting to me how fads that come and go influence the general perception of a product that has true staying power. It was maybe about ten years ago when the HD580 and HD600 and HD650 headphones were often criticized for having the "Sennheiser veil". These criticisms were mostly from people who preferred more emphasis in treble. To my perspective the "veil" was just the very smooth, natural sound from these headphones. But in the present era lots of people, mostly younger people, just can't get enough bass, so now the criticism is that the bass is weak. Well, to be honest the measurements support this perspective, that the bass is weak, but when I listen to them I don't notice this. The reason could be that my hearing is more sensitive to bass. Also worth noting is that the amount of perceived bass depends on the overall volume level, according to the Fletcher-Munson curves. This is why the idea of an ideal or perfectly flat frequency response is a little questionable to start with, because even if the playback speaker or headphone has a perfectly flat frequency response, the listener won't hear the same frequency response that a listener at the live event or in the studio heard, at the time of recording, unless it happens that the listener is listening at volume level matching the volume level at the live event or in the studio. This is the reason that better preamplifiers and receivers used to have variable loudness controls, back in the day, but this also fell out of favor at some point because some know-nothing with a loud voice went around claiming that loudness controls were corrupting the music fidelity. (The person who did this is a well-known influential person within the audio industry, or used to be, and anyone curious as to who it was will not likely have much difficulty learning who it was, by doing some web searches. Whatever.)
Anyway, in the design of the headphones you bought, the sound power radiated from the front of the driver (toward your ear) is only partially acoustically isolated from the sound power radiated to the back of the driver. There is a fairly large area within the headphones, permitting cancellation between the front and rear wavefronts. As with dipole radiators, the effective attenuation naturally follows a curve that, in dipole speakers and according to Linkwitz, is 6 dB per octave. (The cancellation/attenuation increases as frequency decreases, owing to the way that directivity varies according to frequency.) I expect that the bass attenuation curve is much different in these headphones, even without factoring in the acoustic impedance (the membrane) that is applied to this area to limit and control the amount of bass cancellation. The more acoustically transparent the membrane is, the greater the bass attenuation. Sennheiser has fine-tuned the acoustic transparency of the membrane over the years in order to achieve the bass level that they deem optimal for this headphone. The measurements suggests that the engineers who own the design are not fond of bass, but perhaps there is something not quite right with the measurements, or perhaps it has to do with anticipated listening level, or something esoteric that they do not share publicly.
In any case, it is apparent that if the membrane is made less acoustically transparent, the bass will increase. I will caution you to proceed at your own risk. If you ruin your headphones, don't even think about blaming me, because all that will do is make me laugh. You can find videos on YouTube that show how to disassemble the headphones. Be especially careful not to damage the driver or the fine wires that connect the coil to the little springs that provide the contact to the cables. Those wires are extremely fine, and if you break one of them, you will likely have to replace the driver. Here is a link to one of these videos:
The obvious questions: (1.) What kind of material should you add to the barrier? (2.) How should you attach it? (3.) How much of the existing barrier should you cover?
The obvious answer: You are on your own.
If I were doing this, I would maybe start with a thin sheet of open cell foam, partly because thin sheets of open cell foam are easy to come by. I would probably start by covering all of the barrier, and I would attach it using the least intrusive means that I could think up. If I used glue, I would use small dabs in carefully chosen locations. Obviously, the locations for applying glue are the same locations where the existing barrier is bonded to the supporting framework. You obviously want for it to be fairly taut so that it doesn't flop around. Note that you could fill the space between the barrier and the outer grill, but the effect of doing this would not be the same as the effect of making the barrier less acoustically transparent. I.e., you want to use a thin sheet of foam or a cloth, placed directly against the barrier. Another approach might be to use a thin, stiff piece of cardboard with holes strategically cut through it. The total area of the holes would matter, but the size of the holes likely would not. I would try to avoid covering the area directly to the rear of the driver. I would also work entirely on one side and not do anything at all to the other side until I had something I liked.
Finally, I will say again that if you ruin your new headphones I don't even want to hear about it. I'm only pointing out something that is obvious to me and that has been obvious since the day I bought the HD-580, nearly three decades ago. Proceed at your own risk.