[Assorted]

I have no idea how audio circuitry works, but can I assume volume is equal to volts? In that case...

Have both sources play white noise, and stick in a voltmeter. Adjust accordingly so that they're both have equal output (in this case, using headphone out on portable DAPs).

Can I do it? Would I risk anything? It would be so much easier than using an SPL meter everytime.

 

[Assorted]

So I'm assuming this doesn't work?

 

[Fitz]

I've done this before to do a simple check for channel imbalance in a stereo amp... play a sine wave and measure the AC voltage on each channel's output.