Simple. Don't need a voltmeter if you have VU meters.
so:
If a system is set for unity gain throughput, any place you measure it (from the first meter's reading to the last meter's reading) should be the same. Unity gain examples: 0dBu in 0dBu out or -10 dB in, -10 out, +4 in, +4 out. Insertion loss can be measured in dBu (displayed on the meter) as well as dBv (references voltage). On either scale, the readings need to match for good gain structure.
If I feed a 1K signal into my preamp and set the preamp to output a level of -10dB(u, as I'm taking it off the meter), I should read -10dBu on the power amp's meter if I have my gain structured for unity. That's the result I get when the CR1 is not inserted in the circuit. However, when doing the the same thing with the CR1 inserted, (even before employing the filters) I read about 4dB less at the power amp.
I'm losing 4dB when the crossover is inserted in the system. 4dB insertion loss. Measured as dBu.
dBv, dBu, etc.