Vocals tend to boom when boosted around their fundamental frequencies, with 'Exit Music (For A Film)' by Radiohead and 'Peach' by Prince exemplifying the effects of such processing. Plosives and handling noise can be reduced by cutting below about 100Hz, though this improvement needs to be balanced against the side effects for the rest of the vocal sound.
Nasality can be addressed at around 1-3kHz, while the 4-5kHz range can be boosted for more presence. If the extremes of the frequency spectrum are removed to leave only these tonal areas, then you can get the 'telephone' special effect which will be familiar from tracks such as White Town's 'Your Woman' and Space's 'The Female Of The Species'.
The 7-12kHz region will emphasise sibilance and breath noise, a characteristic trait of Sheryl Crow's voice in 'If It Makes You Happy' and Seal's backing vocals in 'Kiss From A Rose'. The 16-18kHz region, on the other hand gives a crispness to vocals that helps pull out details in the recording. The Radiohead vocal, while boomy, also has a lot of energy in this region as well. Shania Twain's voice on 'Still The One' is another example of a vocal with lots of extreme high-frequency energy, though this is more likely to be due to psychoacoustic enhancement; a common processing choice for pop vocals.
Voice - EQ Fundemental
Most voices have little below 100Hz so use low cut to remove unwanted bass frequencies.
Boxy at 200 to 400 Hz.
Nasal at 800Hz to 1.5kHz.
Penetrating at 2 to 4 kHz.
Airy at 7 to 12 kHz.