Introduction:
Superlux HD661 is Superlux's cheap clone to the Sony's legendary MDR-7506(Minus the Audio Technica Wing-Type headband). Due to Superlux carelessly aiming for the MDR-7506 frequency response(Since they could also aim for Balanced Frequency response because Sony designed the 7506 with refined mids), you will have to deal with the "Treble Cannon" nature of the HD661. Songs that has a very forward treble will be unbearable with this cans that you might throw them away.

People are usually into Superlux because of their High Price to Performance ratio that headphones within the similar price range are struggling mostly, and the price that is just so cheap. But on some of their units, they have unbearable treble, they sound like listening to good headphones through razor blades, as said on InnerFidelity. Some people say that Superlux headphones has a sound signature that tries to be Grado as far as highs are concerned. People are also sometimes saying Bad things to Superlux because they have comfort issues(well, you pay cheap so deal with it).

In my Perspective, The drivers of Superlux headphones has potential to sound good enough and they do sound good already enough but they need some refinement on some acoustic chamber refinement. For short, this calls for a mod.

Disclaimer:
Modifications may void your warranty, I will not be responsible for the damages on doing this mod
Do not do this unless you trust your modding skills, or at least you have money to throw cash into for another set of headphones.
This is now a quite perfected Mod to be neutral for monitoring purposes.
Changes in tonality of your headphone will be expected, and this may not be for your liking. No measurements will be provided(Since I am on a budget and has no money for testing equipment), But if you really want to get away with the high frequencies piercing you, you have come to the right place. Don't worry, the mod is easy as 1,2,3.

Materials needed:
-Audio Technica ATH-M50x Replacement Pads
-Screwdriver/Thumb Tacks/Small stick
-Scissors

Instructions:
1. Remove the stock earpads. simply twist and pull to remove.
2. Using a screwdriver, pierce two holes on the ear disc
3. Cut out the veil of the stock pads
4. Put the veil on the back of the M50x pads
5. (Optional) If you still find them to be still too sparkly, you may consider adding more damping materials (other than the veil from your stock superlux pads) to the driver. The preferred materials would be the veils from dollar store on-ear headsets, and embroidery cloth. Paper towels are not a good damping material for the headphones because the piercy treble becomes a slamful treble, especially on the cymbals.
6. Put the m50x pads on the headphones

Reverting the Hole mod(In case you don't feel the mod is useless):
1. Grab a scotch tape, and tape it in the holes of the ear disc.

Notes:
-The mod is tested with the ff. Tracks(These tracks are the worst case for headphones that has High Highs):
-Boring too much holes causes distortion in Treble at High Volumes that it gets louder more.(Tested this when I suffered from minor hyperacusis, and needed some rest, but of course I wanted to strive the perfection of these budget phones)

Waruineko-Ai Shinozaki
Just the Two of Us-Olivia

-My source equipment for testing my Mod is FiiO K1 with AudioQuest Jitterbug

Have a happy Comfortable less sibilant listening