Clarification Gain Setting

Use this formula: P = E^2/R or power equals the voltage squared divided by the ohm load.  If your amp is rated at 600 watts (at 2 ohms), that's the same as the voltage squared divided by 2 ohms.  Multiply everything by 2 and 1200 is the voltage squared.  The square root of 1200 is 34.64... volts.

I understand where your logic stems from, but that would mean that your amp is 100% efficient- which it isn't.  Also, the derived 98... volts is from the assumption that this 44+ amps is travelling through your 2 ohm speaker.

You would go nuts trying to find 98 volts and would kill the speaker in the process.  I suggest you also use the listening method since you're using test tones to set the gains anyway.  Hearing wil pinpoint that moment when clipping occurs, and it's not always at the rated output.  When you hear the tone change, look at the meter for the reading as a means of double-checking the results.  Use ear plugs so you don't end up prematurely deaf.

Remember to do this first with the head unit while the amps' gains are at minimum, so you'll know at what point on the deck's dial to match gains.

To use Ohm's Law calculations for finding amplifier voltage output, look at the Ohm's Law chart of formulas and pick the one that will tell you the third factor when you know the first two:

What two factors do you know, then?  You know that P = 600 and you know that the resistance load is 2.  Find the formula in the chart that calculates voltage using these two factors:  it would be E = SQR(P X R).  So your calculation would be 600 X 2 = 1200; then find the square root of the product 1200, which would be 34.64 volts.  This calculation is for the amplifier's output voltage, not the car's...and efficiency of the amplifier doesn't play a role in this calculation because that factor is already taken into account using the amplifier's rated output.  This is AC volts made by the amplifier and is read at the output speaker terminals that the load is attached to.

When you're using calculations based on rated power, you are ultimately making some assumptions.  In this case you have to assume that the amp outputs 600 clean watts into 2 ohms.  With JBL, it probably does, but with some amplifiers you can't make such an assumption.  That's why you should also listen for clipping.