I've had the htf600's for a couple of weeks now. I like them, for a $30 headphone they're fantastic, but, their midbass bugged me. The mids were clouded by it. Vocals sounded muddy, hollow. So, when in doubt, mod! After all, they're only 30 bucks, so what was there to lose?
I tinkered, and I played and I listened, I experimented with blu-tack and tape. finally i came up with a mod that seemed to clean up the mids and highs, bring out some detail and keep the sub bass while taming the midbass. it basically involves a little blu-tack and a knife.
first step, open it up, remove the dark grey outer ring, (it just pops off if you pry it) then pull the four black tabs outward and remove the silver bit.
now using the tip of a knife, very carefully open up the holes on the back of the driver by removing the semi-porous paper. Be careful not to accidently pierce the driver diaphgram.
Then use the blu-tack to seal most of the vents in the silver piece.
I experimented with different configurations. including sealing them all.I found that leaving two of the small vents at the top open ended up sounding the best without ruining the soundstage.
Now just snap it all back together and enjoy! Clearer mids, more detail, and lots of rumbly bass.not too bad for a 30$ can!