usually, a quality headunit such as Alpine will have an adequate enough EQ to take care of the sound. I set the interna EQ functions along with the basic stuff on my Kicker 5 channel and not concerned about anything else. Just be sure to get a headunit with a good sub section control.....my Alpine 9857 has a very good one.
While that is a good assumption, you need to delve in a little deeper and find out what your needs are. If you want to be able to completely control your system, you might be well to get a processor like the H701 from Alpine. Or the 3sixty.2 from RF. That would be more for competition usage, but if you are looking into personal sound, then the parametric eq in some of the better HU's out there, like Alpine, Eclipse, Pioneer Premier...they will provide all the tuning you would need.
Are they necessary. No. The only time the are necessary, is for competition usage.
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