Moved to Car Audio.
if you have a good head unit(mines stock) do you need an equalizer to control your subs? or does the head unit take care of all that
Generally, you have one of two situations:
1. Your headunit has dedicated subwoofer outputs, which are a pair of RCA jacks labeled "sub". If so, internal to your headunit you'll have options to set what the crossover frequency is (aka which frequencies go to the sub)
2. You headunit doesn't have a pair of RCA jacks labeled "sub". In this case, you generally will use the "passthrough" RCA jacls from the amplifier for your front/rear. You will connect an RCA cable from the passthrough jacks to the input jack of your subwoofer's amplifier. Then, on the amp you can set the crossover frequency of what will go to the sub.
also, while some 'upgraded' factory systems are decent, the vast majority of factory head units are garbage. they usually have underpowered internal amplifiers and a poor EQ curve. they remove the bass at higher volumes to protect the cheap factory speakers from blowing, and to keep the sound 'clean' (good bass requires more power than most factory HUs are capable of producing, cutting the bass out prevents the sound from becoming 'muddy').
you can add a subwoofer to it , but it still will not sound good because the stock HU will reduce the bass at higher volumes (which will also reduce the bass output from your subs). to fix this you need to buy a sound processor to restore the bass and correct the poor EQ curve, and externally amplify the speakers.
i want it so my 6x9 and coaxial speakers act as tweeters and most of the base is produced in the trunk
If you have subs, you will need an amplifier...unless you have cheap wal-mart $10 subs; in which case nothing will help them sound better.
Amplifiers have crossovers built in. A crossover will cut off frequencies to your subs. So you could set the sub amplifier to cut off around 80 hz and it will only produce lower bass.
Your 6x9's and coaxial speakers should be fine running off your head unit. 6x9's are not made to be tweeters. They are full range speakers. It fills in the middle ground. The coaxial speakers you have will have tweeters on them..which will fill in the high end.
If you have after market 6x9's and coaxials you could purchase a 4 channel amp for them as well. This would allow you to customize the frequencies played through them as well.