Bi-Amping: How can I determine relative output?

One of the amps will need to have a gain adjustment. You'll need a sound level meter of some kind if you want an accurate adjustment. I tune my woofer towers by ear but YMMV. 

Amplifiers are voltage amplifiers, and provide a fixed amount of gain (voltage out x voltage in).  There is a convention of 23 or 26 dB, but best to measure it. 

You can use an average multi-meter if you can find a 60 Hz test done.  At this frequency multimeters are spot-on. 

So, play a 60 Hz signal through both amps at the same time and set them so they output the same on the amps.

You don't have to go very loud.  A 5V signal is plenty to tell.

Input sensitivities might be a little off so maybe as you turn up the volume one amp may give more power compared to the other

Buy a test tone CD and a sound level meter

Those old simpson sound level meters are good and cheap these days.

Good luck

You might be better off just using the tube amp, or using 1 good amp.

I like what Erik described above but in the end, you'll have to go with your ears.  My thought is that the songs that sound normal have room for more bass in them.  They might even sound better with the more bass to many or most people.  I would use a bass heavy song and turn it down until it sounds right.  Done.

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.

The issue is that even if you can measure, WHAT are you measuring? Which frequency? The SS and tube amps will respond differently over the impedance curve of your speakers! The SS amp will dump more power into impedance dips and less into peaks. The tube amp will remain more consistent, but generally reduces its output on both peaks and dips. Therefore, the calls to just use your ears are not wrong. However, perhaps you can use your ears more effectively. Here's what I'd do, to "broadband" level-match the tube and A23+, easy peasy:

1. Hookup both amps for VERICAL bi-amping (e.g. tube amp Left; A23+ Right, or vice versa). That's not how you'll run them obviously, because vertical usually requires identical stereo amps. Here we'll just be doing it temporarily to find the A23+'s level. 
2. Ideally you'd have a test track with broadband pink noise (NOT white noise, ouch) that's perfectly equal in Left and Right channels. I have a vinyl source and the HiFi News Test LP has a band that's *perfect* for this. Failing this, use a mono switch (e.g. on your preamp) or mono adapter to absolutely guarantee the played track is identical to Left and Right sides.
3. Play the track. Adjust the A23+ pots (careful to keep them equal) until the image seems as centered as possible.
4. Now reconfigure to a horizonal biamp. 

Note I haven't tried this; just kinda what I thought up now given your situation. I do have an A23 floating around here and the gain pots can be handy. 

Thanks everyone.  Those are some great suggestions and information.

for those more electronically endowed, how would I measure as Erik suggested- what’s the pretty picture look like?

OP: Not sure what you mean, it seems simple enough.

First, find test tones from somewhere in the 50-60 Hz range. Attach a multimeter to one of the tube amp’s outputs. Set the multimeter to read volts AC. Adjust the preamp until the output reads 5V exactly.

Switch to the SS amp and try to adjust the amp’s level settings there to the same.

Erik-

Thanks.  I'm literally, multimeter illiterate. 
"Volts AC" is what I needed.

Thank you!!

PS - 5 V AC corresponds to about 3 Watts at 8 Ohms.  A very low level well within the capability of both amplifiers, but high enough that you can get a solid reading from a meter.

for those more electronically endowed, how would I measure as Erik suggested- what’s the pretty picture look like?

~V

Or a squiggle over or under a horizontal line.

Usually AC is denoted by a small v and DC by a capital V.
By my meter has volts DC as =V when the bottom part of the = is dashed.

if the a21+ is new, it will sound extremely smooth at 450 hours or so like mine did, so that could be an option.  

you can set an amps gain with a test tone and measure the voltage at  the terminals without speakers attached.  the problem is that tube amps require the load of a speaker or you could damage the amp.  

just play pink noise bands above and below the crossover of the woofer and adjust gain so they sound close.  use tracks 47 to 56 on this disk or download

https://sheffieldlab.com/item.php?item_id=52

     There are a variety of Audio Spectrum Analyzers available now on the Net and (looks like) as phone apps.     Seems they (and whatever mic) should be accurate enough, at your crossover freq, to be of service.   

     I've never used either, as I've always owned enough hardware, so: I can't swear to anything.

     Just pay close attention (set on Slow) to the band levels on either side of your crossover point.

     That's if you don't trust your ears.    Female voices/vocals (with which you are intimately familiar) should sound way off, given just a slight disparity in levels.

https://voicemeeter.com/vb-audio-spectralissime-professional-high-definition-multiband-audio-analyzer/