Bi-Amping: How can I determine relative output?

SS Amp has a right and left gain controls.  Using the Fluke, is it possible to measure at the speaker outputs (if so what would I be measuring) in order to get the right/left levels the same?  
 

A little background- the tube amps are plenty for normal use, and sound great.  I’ve been having an itch to run the JBLs at ridiculous levels, so I bought a Parasound A21+ to play with. It sounds great and the combo allows for wall shaking sound, but isn’t as smooth as when using the tube monos. 
 

I noted the gain controls on the A21+, and set out to try a biamped setup.  Given that the speaker is a two way with a pretty low crossover, I though that it may be possible to get decent sound.  With zero adjustment, it sounds much better than I expected.  Currently using the line level pass through on the Parasound, however I have a spare set of the IC I’m using and plan to try connecting to the 2nd output of my line stage.

if there were some way to get both sides adjusted as to some actual metric, not by ear or according to SPL, I’d love to start from that point, and tweak if necessary.

OP:

I gave you the solution! :)  Find a 60 Hz tone, play it and measure the amplifier's output.  The only problem you might have is if your tube amp is has the higher gain. 

Multimeters perform best at 50-60 Hz.

@erik_squires  Voltage out does not equal SPL out. You may get the amps giving the same output but differences in driver efficiency will make the SPL different.

@russ69 - Assuming passive biamping, meaning the original crossover is still in place, SPL matching is done in the speaker's crossover.

Did I misunderstand the OP's problem?

@erik_squires Oh shoot I was thinking about the way I do it with an outboard crossover. You are right.