Can you underpower a Strong amp?

I'm putting together a system and I'm looking at a PG Xenon 200.4 amp and 1-2 JL Audio 10w1v2 subs. 

http://sonicelectrnix.com/item_2750.html   amp

http://sonicelectrnix.com/item_5161.html  sub

I plan on running the components off the front channels and then either one or two subs off the rear two.  My question is, since the components only need 100 watts and the subs need 150 watts, can I prevent all 200 watts per channel from being used?  Does the input sensitivity regulate this and if so can it be set differently for each channel (lower on the comps and higher on the subs?).   I've read how to set input gain using a volt meter but I'm not sure if that's doing what I think I'm trying to do. 

I could go with the 100.4 amp which I'd have to bridge the two rear channels for enough power on the sub but I figured for $40 more I can upgrade the amp to handle any future system (this is for my system).  Any help would be greatly appeciated. 

That amp will be fine for what you are trying to accomplish.

No, you cannot regulate the power being produced by the amp particularly (although backing down the gain will accomplish this to a degree), but the real question is will it harm any of your speakers in this situation? No. You will simply have some head room and will probably be happier to have the extra power to spare in the long run.

If you haven't read this yet, give it a good read as I think it may explain what you are asking.

You can set the gain using the test tone/mutimeter method.  Do a search here or go to JL Audio's websiste.  Set the gain to the speakers' rated RMS power.  This is assuming that your head unit and amp are a good match (the head unit's outputs don't overdrive the amp's inputs).

To second what geepherder is saying, you would be setting the gain so that the amp reaches - not the highest output it can supply - but the highest input allowed to the driver.  The amp will reach that point when the head unit volume is at the highest designated clean output (designated, of course, by you after testing for that point.)

In this case of setting gains and deck volume max, you would listen for the clipped signal first with the deck to find that high point.  Then, with the deck at the high point and speakers unloaded from the amp, measure the voltage output of the amp through the speaker terminals.  Use Ohm's Law calculations to find the target voltage that corresponds to your speaker's RMS and set the amp's voltage output to that point.  The JL flash tutorial is a good visual of that procedure. 

Hahaha.  Smart aleck, or is that a serious question?  If you use the gain to control the amount of power an amp puts out (due to the power limits of the speakers), you can later reset them when you upgrade your speakers for more power which can net you some more volume.  Here's an example of what DYohn is getting at:

Suppose you have an amp that will accept up to a 4 volt input, but you're using an Eclipse deck with an 8 volt output.  Even if you set the gain to minimum, the amp will still produce all the power it's capable of producing.  The solution to this would be to set the gain with the volume on the headunit at a lower setting, thereby limiting it's output rather than using what the deck is capable of putting out.  Let's say you find the volume level where the deck is putting out 2 volts.  You can use that to set your gain, limiting the power output of the amp.