4 ohm dvc subs

You'll get best sound and not only simplest, but best setup by using one of the amps for the two subs.  Subs are wired series / parallel to make a total 4 ohm impedance.  Either amp is rated to make 750 watts into a 4 ohm load at 14.4 volts, and each sub is rated at 300 watts continuous.  Looks like a pretty good setup to me.

Check the woofer wiring application on this site to verify sub wiring.

It's the total power demand, not the impedance (or resistance) of the amplifier's load, that would make the alternator work harder.  It doesn't matter whether its 1 ohm or 8 ohms.  Your car's system will have to supply up to 900 watts of power for these subs if connected as I described above.  (...let's say a peak at 750 watt rating plus 20% inefficiency factor...) 

But of course the alt doesn't supply watts, it supplies current.  900 watts at a voltage of 14.4 is 62 amps.

You're right about the amp being a bit less efficient at a lower resistance.  I didn't consider that, but at the same time I don't consider that an important enough issue to make note of.  The quality of sound and life of the amp are a more important concern for me.  Anyway, it's really no less accurate just to consider power alone because amp efficiency is just surmised to begin with.

While replying, I did do a google search yesterday for that sub model because I didn't know for sure what power it could handle, and I found a review at cnet and a listing at Crutchfield that said it was a 300 watt sub.  You can get a lot of sound out of an efficient 15" 300 watt sub with 1 watt, and I noticed the price topping out at $140, so I thought that what I saw might be correct.  But I guess you are saying this sub handles high power?  What is its continuous power rating?  You're not applying a "peak" sub rating to an amplifier's RMS power rating, are you?

I agree that both subs and one amp would be the way to go.

375 rms will be more than enough to get the most from those JBL's.  Add to that the fact that those big RF amps are under-rated, you're almost guaranteed 400w+ per sub.

There's really no 2 ways about it that I can see.

All I can say is, you have two amps that a lot of people would like to have one of.  Those thieves were fools, for sure.

When you compare sub with amp, just go with the numbers.  RMS rating at best vehicle voltage (usu. 14.4) for the amp into impedance load...to...continuous power handling rating for the sub(s).  You first combine the subs, in your case there are two, and find your impedance.  Compare the combined subs' total continuous power capacity to the amp's RMS power rating into that final impedance.  That's all there is.  "Peak" numbers are not used and should be ignored.

A well executed setup with strict attention to setting gain will allow you to use an amp whose power is greater than the subs' thermal handling capacity.  Using test tones and a multimeter to find amplifier voltage output, along with the use of Ohm's Law equations, will lead you to the gain level where you can supply 300 watts to each sub and still have headroom left in the amplifier.  But I think the equipment power ratings are close enough that you could just use the listening test to set gains and keep the subs safe from overpowering.