My headphones are a modified RX300 and a modified RX700. ( Will upload pics later)
I tried a pair of AKG K92s, which I was very disappointed with. Articulate but very very thin sounding so I sent them back.
Next I tried the RX700 based on the positive comments over the years, fresh out the box I thought the SQ was laidback, which resembles many reviews of the cans. Bass was on the thick side, with recessed midrange and high frequency. Transients and dynamics were decent. There was an effortlessness about it compared to my 'squawky' RX300's.
My listening were all done through my TT with Audio Technica AT440Mlb cart and a cambridge DUO Phono and headphone pre-amp.
I followed the modifications discussed here and tried the following:
1 - Damp out the plastic enclosures with sound deadening, bass was still 'thick n full' but slightly tighter that stock, pace, rhythm & timing were still a bit tame
2 - Blocked out the 3 ports above the 'acoustic lens', I wasn't sure this made any difference??
3 - I blocked/partially blocked the vent holes on the driver magnet, this was where bass output were greatly influenced. Drier and bass did not flood the sound field as much.
Those were the initial modifications, however I still wasn't convinced by the rounded off HF and recessed midrange, so after further reading...
4 - Removed the felt around the acoustic lens and punched holes on the underlayer felt.
5 - Blocked 3 out of 4 vents on each driver magnet
Midrange and high frequencies got a bit of a lift, bass muddleness was reduced a bit further.
6 - Stacked a 15mm N52 magnet on each driver
Attack and decay were affected by this magnet stack mod, tidier, can listen at higher volumes, and bass was slightly punchier. Midrange and HF still sounded shy...
7 - Reversed the ear pads, and cut out the fabric screen. Now I think the inside pads are real leather?
Removing the fabric gave the midrange and HF another much needed lift! More articulation and detail to vocals especially. HF better but still slightly tame.
7.5 - Lined some alcantara wrap around the acoustic lens, I think this helped tame a bit of midrange shoutiness on forward sounding tracks. Standing waves on the hard plastic face perhaps?
8 - My next/final modification is to ditch the 3.5 metre cable, long cabling has higher capacitance and is not helping high frequencies. I have ordered Van Damme microphone cable with a low 46 pF per meter capacitance, and a 1.5 meter length. Hopefully this should address some of the high frequency roll off I am still experiencing.
At the mean time I shortened the stock cable to 1.8M, not 100% sure it helped with HF.