I'm seeing several people EQing and looking for ways to add more bass or "warm up" the sound signature of the HE-350. We anticipated this, most entry level headphones have a lot of warmth and bass emphasis, and this tuning is more in-line with the neutral signatures found in mid-fi and up (per the measurements on the product page).
Knowing this is something that folks may want to adjust to their preferences, I figured I'd post a basic HE-350 mod that adds significant bass response and warmth (+3-5db from ~100hz sloping down to +1-2 db at ~600hz if I remember correctly). We have measurements for this mod, but I don't have them on my home computer so I'll post when possible.
This mod is very simple and fully reversible, it requires nothing beyond your HE-350 and a thin tool. I used a spudger from an iFixit kit we had in the office (see pic #1), but anything thin will work.
First step is opening up the back of the headphone housing to expose the back of the driver. To do this, take your spudger (or whatever thin tool you have) and CAREFULLY pry at the edge of the plastic ring at the 1, 11, 5, or 7 o'clock positions, prying gently at each position in sequence until the plastic ring comes out. The plastic ring has four protruding plastic tabs at those respective positions that slot into the housing, holding the plastic ring in place, which holds the black grille in place.
NOTE: These tabs are fragile: you'll be fine if you break one, but more than one and you'll have to find a creative way to keep the plastic ring in place (which matters because it holds the grille in place). The headphone will function fine without the grilles or plastic rings, but you'll have exposed wires and driver parts, clearly undesirable.
With the plastic ring removed, turn the headphone so the grille faces down. It should fall out, revealing the back of the driver.
NOTE: The back of my driver will look different from yours, dont be alarmed. The driver baffle has a ton of holes in it and different configurations of covering/not covering result in totally different sound signatures. The configuration pictured is my favorite flavor (I'll post measurements when possible) so experiment if you want, but don't be alarmed.
In the center of the back of the driver, you'll see a single sticker. Remove that sticker. That's it. Want to reverse the mod? Put the sticker back. I've removed and replaced the sticker many times without compromising the adhesive.
PRO TIP: Try the headphones before you replace the grill and plastic ring. The less you can remove and replace the plastic ring the better, again, you don't want to break those tabs.
Be gentle when you put the plastic rings back, just as easy to break the tabs on placement as it is on removal.
Give it a try and let the discussion know what you think!