I have level-matched a variety of ways in the past. Voltage matching works well for line-level components and power amps, less so for speakers which may vary by more than that in a pair due to their own errors or room interactions (I would check voltage at the terminals and with a decent mic). For components, I typically measured at 1 kHz using a precision RMS meter, then checked test tones at 20/50/100 Hz and 1/2/5/10/20 kHz. More or less. The last few years I was a tech we had equipment that would do wideband sweeps so I just did that instead. Most SS equipment is very flat 20 - 20 kHz so a 20 Hz/1 kHz/20 kHz check will usually do it. Tube components might show more variation depending upon loading.
For any testing we always level-matched to 0.1 dB or less at 1 kHz, and generally that held across the frequency band for all line-level components. Power amps, speakers, ack, usually matched at 1 kHz and prayed.
Sometimes we'd match at other frequencies due to a bump in speaker, room, or speaker/room response.