[FearSC549]

I bought a 75 ohms impedance adapter for my ER-4P to S but I sold my Etymotic the next day because I was in need of cash. I was toying with the 75 impedance adapter by plugging in with random headphones and usually it's all bad results but with the HD 555, it's different. 
 
With the adapter, the bass REALLY improved, almost to the level of my HD 650. I was blown away at the improvement; it sounded almost like a unrefined HD 650. Other than the bass, the mids and highs sound relatively the same(which is why I said unrefined HD 650). This is my second pair of HD 555, I bought only to compare with my HD 535 and 650. The first time I sold it was because they're really not that impressive; I've never liked the HD 555...until now. I really want someone else to try this to see if the same result happens. 
 
An interesting thing is that the HD 555 used to be 120 ohms until they've all been changed to 50 ohms drivers. Technically with the 75 ohm adapter, it's 125 ohms. I don't know if it sounds like the 120 ohms version.
 
Also, do note, I've done the foam removal mod and the soundstage mod(removing plastic grill). 

 

[mikeaj]

HD 555 is like many headphones (but not all for sure) that have impedance that varies significantly over frequency.  According to HeadRoom (as always, measuring headphones is hard and take with a grain of salt), this is the frequency response and impedance over frequency:
 

 

 
 
So it's like 55 ohms at 1 kHz but above 200 ohms around 80 Hz (mid bass region).  The frequency response above is what you would get (give or take) out of a source with minimal output impedance.  But putting a 75 ohms impedance adapter in front, you're essentially increasing the output impedance of the source by 75 ohms.  Thus, the signal sent by the source is divided between the source output impedance+adapter and the headphones.  The voltage you get at each frequency would be Z_h / (Z_h + Z_s) where Z_h is the HD 555 impedance and Z_s is the source output impedance+adapter impedance.  Let's assume the native source output impedance is low like 0 ohms.  0-8 ohms is common though some have much higher.
 
This means that at very low frequencies and most of the higher frequencies, you're losing half the signal or more across the impedance adapter.  Like 50 / (50 + 75) = 0.4 or 40% of the original.  At around 80 ohms, you're getting say 225 / (225 + 75) = 0.75 or 75% of the original.  The impedance adapter is attenuating mid bass less so than the other frequencies, so one effect is that you're pretty much getting an mid bass boost EQ (well, everything else EQ'd down more so than the bass).  Note that the sub bass isn't being boosted, which may sound weird.
 
You're also lowering the damping factor, which would make the drivers less controlled and more prone to ringing.  In theory this is usually bad, but a different sound always may be more pleasing to some people.

 

[FearSC549]

Quote:

HD 555 is like many headphones (but not all for sure) that have impedance that varies significantly over frequency.  According to HeadRoom (as always, measuring headphones is hard and take with a grain of salt), this is the frequency response and impedance over frequency:
 

 

 
 
So it's like 55 ohms at 1 kHz but above 200 ohms around 80 Hz (mid bass region).  The frequency response above is what you would get (give or take) out of a source with minimal output impedance.  But putting a 75 ohms impedance adapter in front, you're essentially increasing the output impedance of the source by 75 ohms.  Thus, the signal sent by the source is divided between the source output impedance+adapter and the headphones.  The voltage you get at each frequency would be Z_h / (Z_h + Z_s) where Z_h is the HD 555 impedance and Z_s is the source output impedance+adapter impedance.  Let's assume the native source output impedance is low like 0 ohms.  0-8 ohms is common though some have much higher.
 
This means that at very low frequencies and most of the higher frequencies, you're losing half the signal or more across the impedance adapter.  Like 50 / (50 + 75) = 0.4 or 40% of the original.  At around 80 ohms, you're getting say 225 / (225 + 75) = 0.75 or 75% of the original.  The impedance adapter is attenuating mid bass less so than the other frequencies, so one effect is that you're pretty much getting an mid bass boost EQ (well, everything else EQ'd down more so than the bass).  Note that the sub bass isn't being boosted, which may sound weird.
 
You're also lowering the damping factor, which would make the drivers less controlled and more prone to ringing.  In theory this is usually bad, but a different sound always may be more pleasing to some people.

Yes, by changing the impedance, the mid-bass especially, will be not as attenuated as before...and the HD 555 is known for lack of bass, which is my I find them more enjoyable. As the damping factor is decreased(or lowering Q), it will be prone to ringing, however headphone drivers are very small and ringing is not as present as compared to full sized loudspeakers. Overall, to my ears, it sounds better.