I'm looking to replace my stock system next January. I plan to run a sub along with 4 speakers. In order to do this I'll be using 2 amps, 1 for the sub, the other for the speakers.

So do I need 3 preamp outputs or just 2 on the deck? Is there an advantage to 1 way versus the other?

You don't have to have 3 sets of pre-outs. If you want to keep the ability to fade by means of the head unit, go with 3. If you don't care, you can get by with 2 (in which case, it's likely the deck's rear pre-outs are used for sending signal to the sub).

the advantage of 3 outputs is so you can control the output of the sub. Most head units with 3 pre-outs have the ability to turn up, down, off and on the sub. also to change the frequency sent to the sub. if you don't care about that, then go for the 2 sets and use the output of the 4 channel amp (if it has one) to send a signal to the sub amp. Good Luck

If you're concerned with fading from front to rear as well as a dedicted sub woofer output you will need 3 RCA outputs - Front, Rear & Sub.

If you're not concerned with fading from front to rear 2 RCA outputs is adequate. More than likely if your deck has 2 preouts they will be labeled as front and rear with rear being switchable from full range frequencies to sub frequencies. In this scenario, you would run the RCAs from the HU's front outputs to a 4-channel amp that would power all 4 speakers. You will want to verify that your amp has the ability to take one input source and distribute it to all 4 channels. The sub would be running off of the HU's rear RCAs.

Another scenario would be to purchase a 4-channel amp that offers RCA preouts on it. This would allow you to attach the HU's front outputs to the amp's front inputs and the HU's rear outputs to the amp's rear inputs. Then you would attach a second amp to the first amp's preouts. This setup would allow you to control front to rear fading but eliminate any control of the sub woofer amp from the HU itself. In this scenario, the first amp's preouts are duplicating the HU's rear output. So, when you controlled front to rear fading from the HU the sub amp would be controlled by the fading of the rear signal.

I would recommend buying a HU with dedicated front, rear and sub preouts. This will give you the flexibility you will probably want in the future.

One more option is using either an electronic crossover, some even have sub level controls Found Here . Or an EQ Found Here

Cool, thanks for the replies. The ones I like seem to have 3 so it should work out fine.

Heres another question. I plan to have the sub installed so I can remove it when need be. This means the amp will be a fixed install. I know you can set the deck to remote power on the amp. Do HUs have controls to turn the amp off full time, so I can pull the sub out. Or do I need a physical manual switch to kill the power to the sub full time?

you need a switch or something. THe few times I take my sub out, I just keep an old philips head in the truck and disconnect the "Turn on lead" from the amp.

If your headunit has 3 preouts, more than likely you will be able to turn the sub off from the headunit. The amp will still be powered when the radio is on, but no power will go to the wire that connects to the sub. Just remember to turn the sub off, otherwise you will blow a fuse on the amp.

I tried searching for a deck that specifically states manual amp power control or some such, no luck.

So you think a 3 preout deck would have this feature versus a 2 preout? OK. I'll check into that. Thanks everyone!