watts at 1.3ohm vs 1 ohm

You have to have another variable:  rated output at 2 ohms would work.   If in this case the 2 ohm output is rated 750 watts:  calculate the 1.34 ohm output by

• find the difference in watts between 2 ohm and 1 ohm.  It is 750 watts.
• 1.34 is one-third of the way from 1 to 2, so divide the difference of 750 watts by 3.  It is 250 watts.
• subtract 250 from 1500 to get 1250 watts at 1.34 ohms.
• if the impedance was 1.67 ohms, the calculated output would be 1000.
• double-check your calculations by noting that when impedance goes up, power output goes down.

Of course, this is just for the sake of having numbers associated with impedance load and may not be true if tested.  If the case were that the amplifier was rated 1000 watts at 2 ohms and 1500 watts at 1 ohm (doesn't truly follow Ohm's Law due to amplifier's high current limitations), the results are derived the same way but would be viewed a little more skeptically.  That's where you could rightly say, "...somewhere between x and y...".

Meaning that it would be pointless to work the amp harder @ 1ohms VS 1.34ohms.