Lion-
Looks like you've got a nice start on the system- congrats!
I wanted to chime in as there is some good information, bad information and misinformation regarding specifics of the audio system setup, amplifiers, clipping, etc. Instead of pointing out what is *wrong* to people who are already convinced they are right, I will simply give you this information so you can use it to help you get your system configured, setup and tuned well.
The JL amplifiers (HD / MHD models) have a feature that adrianp89 and FloJet mentioned. It is called RIPS. This is essentially a fully regulated power supply, meaning that the amplifier can output its full power at a range of power input voltages. (If your boat is running and has 14.4vdc power supplied to the amplifier it will provide the same output power as if it is off and has 12vdc or lower, up until the point there is too much voltage sag/drain that the amplifier will perform poorly, or shut off for under voltage. The JL’s also output (about) the same power at each speaker impedance load you apply to a given channel. Those are nice benefits. The drawbacks are that with an unregulated power supply, you’d have the potential to make more power at lower impedances or when bridging. Unregulated power supplies though will not produce the same constant output power once the input power drops below 14.4vdc. This is why you will see amplifiers with different rated output power based on 14.4v or 12v input.
Now, for impedance loads to an amplifier… Amplifier channels are “stable” to certain impedance loads. You can ALWAYS connect a higher impedance load to amplifiers channels without risk of damaging the amplifier, but you should not attempt to connect an impedance load which is lower than the rating of a given amplifier channel. The impedance load could be a single speaker, or multiple speakers wired together. What is important is that the impedance load presented to the amplifier (by whatever number of speakers are connected to the channel(s) ) is within the spec allowed by the amplifier manual. Speakers come in a multitude of impedance ratings. Most car audio speakers are 4 ohm, with occasional 2ohm or 8ohm impedances. Most home audio speakers are 8ohm with the occasional 4ohm, 16ohm and higher impedances. You can connect speakers together in series/parallel configurations to combine their impedance loads which would then be presented to the amplifier. Of course, the amplifier channel can only put out so much power so connecting many speakers to a channel (even if it is safe from an impedance standpoint) is not always a good idea because it may not yield enough output volume.
Bridging… We know that amplifiers can produce the same output on a channel at multiple impedances (with regulated power supplies) OR they could output more power at lower impedances (with unregulated power supplies). Another thing that many amplifiers allow you to do is to “bridge” channels, or combine the output power of 2 channels into one. *Typically*, when you bridge channels, the amplifiers safe impedance load that can be connected is doubled, so you cannot connect loads too low to it. *Typically*, when you bridge 2 channels, the amplifier makes at least double the output power into a single channel. Adrianp89 was mentioning the value of this when you may want to have more output power available for a given application. For example, maybe you would have a speaker like a subwoofer or a large woofer and you want to be able to provide more output power to it than a single channel can provide…bridging allows you to do that.
Clipping and signal voltage… I have only once measured a source unit that did NOT clip on any output at a range of frequencies at its max volume. And that was a VERY expensive unit no longer produced. Using an oscilloscope allows you to find the clipping point of your source unit so that you will know to keep the volume below that setting for the cleanest sound. Your goal is to get a clean signal with enough voltage to fully drive the amplifiers being connected. It is important to note that what adrianp89 said is very much correct. The gain is to adjust for the input voltage provided to the amplifier. All amplifiers require a certain amount of input voltage in order to fully drive them to maximum output power. (Each amplifier model is different in how much is required). If your signal voltage from your source unit is low, then you may need to adjust the gain up on the amplifier to account for the lower input signal. If you have a very high input voltage, chances are you won’t need to adjust the amplifier gain up much to reach maximum output before clipping. A higher voltage signal is a good thing because it provides a higher signal to noise ratio. It is important to note that amplifiers have a rating for how much input voltage signal they can handle- you *can* have too high of a signal which could damage the amplifier. (So be careful with line drivers as some produce a LOT of signal voltage)
If you plan to use a separate DSP unit (some source units have enough DSP power built in to do it on their own) for tuning the system, that would go in between the source unit and the amplifiers and you would still want to ensure that the DSP output isn’t clipping. You would also then ensure that the gains on the amps are set based on the input voltage the DSP unit is sending them.
Best of luck in your pursuit for great sound!
