Install a tone generator on your smartphone and connect it to the amplifier:
https://play.google.com/store/apps/details?id=com.keuwl.functiongenerator
Or, alternatively, this frequency sweep wav can be used:
https://www.audiocheck.net/testtones_sinesweep20-20k.php
Borrow a second phone, install a dB meter and hold it at ear height on the listening position:
https://play.google.com/store/apps/details?id=com.keuwl.functiongenerator
The dB meter is probably not accurate, but we’re looking for relative differences, not absolute accuracy. Besides speaker placement and possible measures to improve room acoustics, the listening position may need to be changed to avoid sitting on acoustic nodes or antinodes of important frequencies. All this can be perfectly measured with the two free apps.