For almost a decade I've had a modded HD555, and another one that's not modded. I have never been able to tell a difference in audio output between the two. By contrast, moving to strictly better gear (as when I got the first HD555, or when I got the HD6XX that's replaced them both for music listening) is an immediately perceptible step change.
Basically, the foam mod is nonsense. I get why it was popular, especially in audiophile circles where snake oil is always at least on the menu and often just is the menu. (Green marker around the rim of a CD, anyone? It makes the bits sound warmer!) At least in the case of the foam mod there's a physically plausible mechanism, which puts it head and shoulders above most audiophile nonsense. But it still does not make a perceptible difference in practice.
Basically, the foam mod is nonsense. I get why it was popular, especially in audiophile circles where snake oil is always at least on the menu and often just is the menu. (Green marker around the rim of a CD, anyone? It makes the bits sound warmer!) At least in the case of the foam mod there's a physically plausible mechanism, which puts it head and shoulders above most audiophile nonsense. But it still does not make a perceptible difference in practice.