Bi-Amping: How can I determine relative output?

@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!!