Does anyone know which equalizer preset one should use in iTunes?

Depends on the music you have and the sound you want to get. Personally I use my own custom setting that I borrowed from Winamp’s preset called “Large Hall”. It works excellently for classical and seems to work fine for most of the music I listen to (which in terms of genre is all over the place).

It is mostly pop and soft rock. I’d like for them to be loud. Same with movies. Some songs are very soft, even at loudest volume level.

It depends on the music which is OK because iTunes will let you assign a different preset for each track if you want to (its on the options tab when you “get info”).

I adjust mine constantly based on teh frequency response of the speakers/earbuds I am using and for speakers the space, as well as for the music style and the quality of the recording.

There is no “one size fits all” setting, and if there were, I doubt it would be any of the presets. I love 5/7/9-band equalizers mostly because presets suck

I’ve never trusted those presets and still don’t. If you’re sure you need EQ I’d make the adjustments manually.

I listen to a lot of old mono jazz recordings that lack sufficient bass and high end, so I tend to boost both ends a bit, especially the lower end. But if you go too low it will sound “boomy” or—even worse—overload the speakers or overdrive the subwoofer. If the bass boost overlaps too much with the midrange it will sound “boxy” or “nasal.” Too much high end will sound screechy or strident, and if the recording lacks those frequencies to begin with then all you do is boost the noise levels, which increases hiss.

In general too much EQ is worse than none at all. Be very judicious in how you set it. When in doubt keep it flat. Also keep in mind that both old and new recordings are often boosted when the master is mixed or transfered, in which case you might actually need to CUT one end or both ends to maintain fidelity.

The “classical” setting is usually fairly flat, so least likely to ruin the sound. The “loudness” setting is meant to compensate for low volume levels, to overcome a limitation in human hearing. Sound systems used to routinely put loudness compensation circuits into analog hardware, but that’s hard to find these days.