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).
Edited by FearSC549 - 5/19/11 at 10:06pm