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This is my Superlux 681 inside the cup, 668B look the same, you can see i add some blutack to the back of magnet of driver and also add some blutack to the rear . This mod is to add some mass to the cup because the 681 is quite light but have big bass that make resonance. This mod tighten the bass of 681, i think the 668B doesn't need this mod.
There are some other mods for 681 can apply for 668B:
-AKG velur pad: increase comfort
-Filter mod:
http://www.rock-grotto.co.uk/HD681.pdf, if you think your 668B has too much sibilance you can do this mod but with different value of capacitor, inductor and resistor.
-Filter box: instead of opening your cup and mount the filter to the driver, you can make a filter box and use with your headphone whenever you want and it's also easier to change resistor to vary the amout of high you want to tame
-dampen mod: Use some fish tank filter wool to damp the cup, this mod can vary bass quantity (lower) with different amout and tighten bass
-Recable with shielded OFC cable: top end more refined, more sense of air, soundstage more accurate
-Mick ring mod: change the outer dull red of the 681 to alu ring that make this can look gorgeous
-Pence mod: glue each side a 2 pence coin to add mass ( it fit perfectly), this mod is a easy but less effective verison of Mick ring mod
-Tissue mod: use soft tissue to prevent driver from touching your ear, you can use rubber tube too, this mod is so great, totally comfortable but the downside is it can ruin the sound quality, you must try yourself different material and thickness to make it both comfortable and sound great.