There are a couple of different ways to approach it mate. Basically only limited by your goals, budget and knowledge.
First off, you require a four channel convertor (or more, but unnecessary) Two channels for the front speakers and two channels for the rear speakers. The subs will take their feed from the rear channels by splitting your rcas, using the pre out on one of the amps or using a loc with a dedicated sub output.
Your entry level LOCs are basically isolation transformers and require no power. They simply present the headunit with a 300-3000k ohm impedance instead of the speakers 4 ohm. This drops the power of the headunit significantly and it then outputs it into a level more suitable for the amplifier. When just adding a sub to a system and nothing else... They are perfect. With that said, many of my budget conscious customers use these in full systems when cost is a concern. They come in a couple of different types.
Full Passive: These are the simplest of all and a four channel is about $40
AP3044 4 channel hi/lo audio level | Aerpro
Then you have Partially Active. Not much of an advantage for just a sub but they do allow better quality by feeding the reduced signal through a pre-amp before sending it to the amplifier. Some nicer units will actually provide an automatic trigger wire when it detects a signal. This is useful in cars like the VE where you can play the stereo without the ignition on. The one I've listed below does this.
APHL4 4 ch. hi / low level converter | Aerpro
After this, you start to head into your more complex realm... The first thing you will start to notice in the high end units is what they call Signal Summing. The VE does NOT require this at all. In some other cars you cannot take your feed from the headunit. It may be data (digital) or simply two channel with a data connection to the amp. The amp then has multiple outputs for each corner of the car. It will have a seperate output for each tweeter, for each mid driver and each subwoofer. In this instance, all of those outputs have a HPF, LPF or Bandpass filter applied to them inside the amp. Meaning a full range signal cannot be achieved. Nor can you simply grab all the speaker wires and join them together. Kiss the OEM amp good-bye. A LOC capable of signal summing will allow you for example to feed the tweeters, mid and lows and created a single pair of full range outputs. Hope this makes sense lol.
Here is an entry level Signal Summing Line Output Convertor... you don't need this.
APU8SUM Hi-lo level signal summing | Aerpro
Others in the Mid Range level offer Sub control (RUX knob) With this unit, you can run your four inputs into it, then take a feed from your rear inputs, send it to the sub input and have a dedicated sub output on the LOC and if you have look... it has an EQ on it. Other models will allow four channel input converted directly to 6 channel output. Some have the bass controller optional... some include it.
APHL6EQ 6 ch. hi / low level conevrter | Aerpro
http://tdj.com.au/firmware/Aerpro/APHL6EQ.pdf
Here are some more examples...
Audison bit Ten (considered one of the best. I say it's definetly awesome, but as for the best... it's all personal taste)
http://www.audiocontrol.com/t34/176...Converter-with-Equalization-and-AccuBASS.html (one of my favourites in regards to bang for buck)
CL441dsp - Car Audio - OEM Interface - OEM Integration - JL Audio (offers automatic EQ tuning)
After this.. we go into the full blown sound processor units. These are the bees knees. They allow on the fly tuning from Mobile iOS/Android device, bluetooth, laptop or video output to headunit and then IR remote control. These guys also cost real money... as you can imagine. Additional features often include things like time delay control, bass enhancers, acoustic alignment from left to right and often auto tuning when a microphone is plugged in. Some also offer bluetooth music streaming!
and... if so inclined... 5.7-7.1 surround sound.
Audison bit One (I love this for ultra high end vehicles as it accepts fibre optic input.)
3SIXTY Digital Processors - 3SIXTY.3 - Rockford Fosgate® (Possibly the most feature packed unit at a price)
Alpine Electronics of Australia (offers quite a bit and not a bad price compared to the rest)
After this, you can add other options such as active crossovers, line drivers and bass enhancers
First off... I recommend none of these brands FOR YOUR APPLICATION. I have no idea what you want to achieve, how much you understand and how big your budget is. I simply provided some insight and some examples. You make a wrong purchase... thats on you!
As for which is the best unit, whoever says that X is the best one because they've got one and it's great... are not helping you. Unless they have the exact same requirements as you, they are not helping. It is merely a suggestion. This forum has many experts in ICE, many wanna-be experts in ICE, many who freely admit they have no clue but offer what they know (which is good) and then you also have your neighbour who has a mate who's brother in-law used such and such and hit 170dB in his Lancer with a Sony Xplod subwoofer.
Its upto you to decide who is who! haha... good luck and if you have further questions, fire away! You are more than welcome to PM me and I will respond (usually!) but remember, you will only get my opinion. Ask on your thread and you get a lot more opinions. Some good, some not so good. Hopefully the former
Peace!
